CRP 201: PLANT TAXONOMY – LECTURE NOTE 2

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INTRODUCTION

Plant taxonomy deals with the description, identification, and naming of plants, and their classification into different groups according to their resemblances and differences mainly in their morphological characteristics. Plants are not only numerous but they are of varied types,- and it is not possible to study them unless they are arranged in some orderly system.

The objective of systematic botany or taxonomy is to describe, name and classify plants in such a manner that their relationship with regard to their descent from a common ancestry may be easily brought out.

Classification denotes the arrangement of a single plant or group of plants in distinct category following a system of nomenclature and in accordance with a particular and well established plan. The ultimate object of classification is to arrange plants’ in such a way as to give us an idea about the sequence of their evolution from simpler, earlier and more primitive types to more complex, more recent and more advanced types in different periods of the earth’s history.

Identification, on the other hand, involves referring an individual specimen to a previously classified and named group (Jardine 1969a). After groups of organisms have been classified, names must be given to these groups so that communication about particular units will be facilitated and so that continued progress in classification can be made (Hitchcock 1916). The naming of groups of organisms and the rules governing the application of these names together are called nomenclature.

 

The taxonomy hierarchy

In order for ranking to be achieved, a hierarchy of categories must be provided into which taxonomic units can be placed.

1 For organisms, such a structure is called the Linnaean hierarchy, after the Swedish botanist Carl Linnaeus (e.g., 1753, 1754), who first consistently used many of the categories we now employ. With inanimate objects, many different types of hierarchies, or sets of classes, are available for use. For the biotic world, we use principally one, and this rigidity prevails for at least three reasons:

  1. for purposes of efficient and exact communication on a worldwide basis, one standard hierarchy is essential;
  2. one particular category, the species, is fundamental to our understanding of the organization of organic diversity, and, therefore, all other categories in the hierarchy relate directly or indirectly to this level (this relationship imposes limitations on the numbers and kinds of categories available); and
  3. it is assumed that all life originated in the same general way through

evolution by natural selection, and, therefore, the resultant units of

diversity recognized should apply equally well to all or any part of the living world

 

 

Units of classification

  1. Species: By the term ‘species’ we mean a collection of individuals (plants or animals) which resemble one another in almost all important morphological characteristics-both vegetative and reproductive-so closely that they may be regarded as having been derived from the same parents. The species is the fundamental category of the taxonomic hierarchy. Species are the “building bricks” in biological classification from which concepts of higher and lower groups are developed.

Thus all pea plants constitute a species. Similarly all mango plants constitute different and distinct species. Occasionally, owing to ‘variations in climatic or edaphic conditions, individuals of a species may show a certain amount of variations in form, size, colour and other minor characteristics. Such plants are said to form varieties. A species may consist of one or more varieties or none at all. A cultivated variety of plant is referred to as a cultivar.

  1. Genus. A genus is defined as a group of species which from likeness appear to be more nearly related to each other than they are to other species. A genus is a collection of species which bear a close resemblance to one another in the morphological characters of the floral or reproductive parts. For example, banyan, peepul and fig are different species because they differ from one another in their vegetative characters such as the habit of the plant, the shape, size and surface of the leaf, etc. But these three species are allied because they resemble one another in their reproductive characters, namely, inflorescence, flower, fruit and seed. Therefore, banyan, peepul and fig come under the same genus, and that is Ficus.

Binomial Nomenclature. This is the scientific method of naming species of plants or animals in two parts: the first refers to the genus and the second to the species. This system of naming plants or animals with a binomial was first introduced by Linnaeus in 1735 and the rules for its final adoption were drawn up by the International Botanical Congress held at Vienna in the year 1905. The name of the author who first described a species is also written in an abbreviated form after the name of the species, e.g. Mangifera indica Linn. Here Linn. refers to the author, Linnaeus, who first described the plant.

  1. Family. A family is a group of genera which show general structural resemblances with one another mainly in their floral organs.

From class, organisms are placed into an Order and then a Family. Using grasses as an example from the plant kingdom, they belong to the order Poales and the family Poaceae.

 

Guiding rules in the naming of plants.

The rules for naming plants are very specific. The International Code of Botanical Nomenclature (ICBN) contains authoritative rules on the correct way to name plants, as well as groups such as algae and fungi, which have traditionally been considered plants in a broad sense.

  1. For a plant name to be accepted, it must be validly published. For any new species (or genus) described before 1953, “validly published” could mean anything from publication in a newspaper or catalog to publication in a respected scientific journal or other professional work. Since 1953, all new names must be published in accepted scientific publications.
  2. In addition, all new species (or genus) descriptions must include a complete description in Latin, often called the Latin diagnosis. All names of taxonomic groups are treated as Latin, regardless of their source. Proper names and non-Latin words must be Latinized, following specific rules in the ICBN.
  3. In addition to being validly published, a type specimen must be identified. A type specimen is a preserved plant specimen that is designated by the author as the best representative of the new species.

Sometimes two or more plant taxonomists may inadvertently describe the same species, giving it different names. When this happens, the earliest validly published name is given priority and is considered the correct name; any other names are called synonyms

  1. Species names always comprise the genus name, with the first letter capitalized, followed by the species epithet, which is not capitalized.
  2. Both names must be either italicized or underlined to denote the name as a species name.
  3. A complete species name is also followed by the name of the author who named it. Author names are often abbreviated, and many author names have official abbreviated forms. An example of a species named by Linnaeus is Brassica rapa L. (the L. stands for Linnaeus). The author’s name should not be italicized or underlined.
  4. Once a genus has been referred to in a scientific paper, later references to species within the genus can then be written with the genus abbreviated to just the first letter and the author’s name is left off: for example, Brassica rapa L. becomes B. rapa.

In a species with a lot of variability, subspecies and varieties can also be described. Some plant taxonomists consider subspecies to be of higher taxonomic rank than varieties, whereas others treat them as equivalent. Often particular taxonomists will use only one of these ranks to describe taxa below the species rank. Any species can be split into two or more varieties or subspecies. The variety or subspecies that contains the type specimen is always considered the typical variety or subspecies.

For the sake of simplicity, italics are now often used for taxonomic groups higher than the genus, all the way up to the phylum. However, strictly speaking, only the genus and species names are italicized

Naming Rules: Above the Genus

Above the genus the type concept is used to determine correct names. All family names must be derived from a genus name within the family. For example, the rose family is called Rosaceae, which is derived from the genus Rosa, and the lily family is called Liliaceae, which is derived from the genus Lilium. Exceptions to this rule are only allowed when acted upon by the International Botanical Congress.

Taxonomy of important crops in agriculture

Taxonomy Maize Rice Cowpea Soybean Cassava Sweet potato Oil palm Cocoa
Kingdom Plantae Plantae Plantae Plantae Plantae
Division Magnoliophyta Magnoliophyta Magnoliophyta Magnoliophyta Magnoliophyta
Class Magnoliopsida Magnoliopsida Magnoliopsida Magnoliopsida Magnoliopsida Dicotyledons
Order Poales Poales Fabales
Family Poaceae Poaceae Fabaceae Leguminosae Euphorbiacea Convolvulaceae Palmae Steculiaciaceae
Genus Zea Oryza Vigna Glycine Manihot Ipomea Elaies Theobroma
Species Zea mays Oryza sativa V. unguiculata Glycine max Manihot esculentum Ipomea batatas Elaies guineensis Theobroma cacao

Meaning of nomenclatures used for the crops

Plantae—organisms that have chlorophyll ‘a’ and ‘b’ contained in chloroplasts and show structural differentiation

Magnoliaphyta (formerly Anthophyta)— vascular plant with seeds and §owers; ovules enclosed in an ovary (the angiosperms)

Magnoliopsida (formerly Monocotyledones)— embryo with one cotyledon; §ower parts usually in threes, stem with scattered vascular bundles

Poales (formerly Commelinales)—monocots with fibrous leaves, reduction and fusion in lower parts

Poaceae—hollow-stemmed monocots with reduced §owers, fruit a caryopsis; the grasses

Zea—robust grasses with separate staminate and carpellate lower clusters; caryopsis is fleshy Zea mays—corn

THE FAMILY: POACEAE

Maize

General description

Maize is a versatile crop grown over a range of agro-climatic zones. In fact the suitability of maize to diverse environments is unmatched by any other crop. It is grown from 58°N to 40°S, from below sea level to altitudes higher than 3000 m, and in areas with 250 mm to more than 5000 mm of rainfall per year (Shaw, 1988; Dowswell et al., 1996) and with a growing cycle ranging from 3 to 13 months (CIMMYT, 2000). However, the major maize production areas are located in temperate regions of the globe. The United States, China, Brazil and Mexico account for 70% of global production. India has 5% of corn acreage and contributes 2% of world

production (faostat.fao.org, 2008)

Maize cob

Taxonomy

Maize belongs to the tribe Maydeae of the grass family Poaceae. “Zea” (zela) was derived from an old Greek name for a food grass. The genus Zea consists of four species of which Zea mays L. is economically important. The other Zea sp., referred to as teosintes, are largely wild grasses native to Mexico and

 

 

Central America (Doeblay, 1990). The number of chromosomes in Z. mays is 2n = 20

Taxonomy of Maize Kingdom Plantae Subkingdom Tracheobionta Vascular plant Superdivision Spermatophyta Seed plant Division Magnoliophyta Flowering plant Class Liliopsida Monocotyledon Subclass Commelinidae Order Cyperales Family Poaceae Grass family Genus Zea Species Zea mays Sunspecies Z. Mays supsp. Mays

Plant Morphology: The plant is 1 to 4 meter tall. Approximately 30 leaves is present on per plant with erect stalk-like structure. Is a meristem. Sheath surrounding the stalk. Expanded blade by blade joint or collar. It has nodes and internodes.

Leaves Morphology. Leaves are broad and a single leaf. Leaves are arranged in two vertical rows on the opposite sides of an axis. (distichous). Long, large, alternate, parallel veins.

Root Morphology: Fibrous root. Brace root form at the bottom of the stalk. support the plant and scavenge top levels of soil for moisture and nutrients. Seminal root ~ nodal roots originate from scutellar node. sustain seedling development by virtue water intake.

Reproductive Morphology of Maize: Male & female inflorescences are located at different part. Male inflorescence is called tassel. Female inflorescence is called ear. Maize pollen dispersion is by wind. It is an annual plant. Anther – male reproductive part of the corn plant, consists of several small branches, along which small flowers grow. The flowers release pollen grains, which contain the male sex cell. The ear is the female reproductive part of a corn plant. Ears develop from “shanks,” which are stalk-like structures that grow from the plant’s leaf nodes. A corn plant may produce many ears, but the uppermost ear will grow to be the largest. The ear consists of a cob, eggs that eventually develop into kernels and silks. Pollination occurs when pollen from the male tassel falls on the female silks.

 

The maize plant

 

Variation within a single maize landrace showing the effects of human selection for kernel shape as well as ear size and shape

Corn Seed of Maize A protective sheath enclosing the shoot tip and the embryonic leaves of grasses. The triploid nutritive tissues formed within the embryo seed plants.

Male flowers Section through pair of male flowers

Its Structure & Function Structure Function Stamen Pollen producing reproductive organ which are referred to as androecium. Stalk Also known as filament, the part of the stamen the anther develop. Anther The terminal part of a stamen which the pollens are produced. Style Slender part of the pistil, situated between the ovary and stigma. Stigma. The receptive apex of the pistil of a flower on which pollen is deposited. Sheath Part of leaf originating from the node and running parallel to the culm or stem. Ligule A membrane located between the culm and the leaf blade.

 

Life Cycle of Maize (a) The haploid gametophytic stage . (b) Male gametogenesis – the microspore (pollen grain) undergoes two mitotic divisions to produce a three-celled gametophyte. Female gametogenesis -the megaspore undergoes three free-nuclear divisions followed by cellularization into seven cells, to produce a haploid megagametophyte. Egg cell becomes the embryo and fertilization by the second sperm of the central cell with the two polar nuclei produces the triploid endosperm (c) Shows the endosperm and embryo, both of which are products of the double fertilization. The outermost layer of the endosperm is the aleurone. The entire kernel is covered with a diploid maternal tissue called pericarp.

 

Corn has a life cycle of 120 to 150 days. It is best to plant after there is no more freezing and it is above 50 degrees. It will grow to 3 to 10 feet tall during its cycle. There are several types of kernels to use such as yellow, red, orange, black and bronze. For the corn to begin to germinate it has to have lots of water, 30 percent to be exact. The kernels must be planted 2 to 3 inches.

 

Root Rice forms a fibrous root system consisting of seminal, nodal and lateral roots. As the rice grain germinates, the radical

 

R

ice

 

Taxonomy

 

Rice belongs to the genus

Oryza

and the tribe Oryzeae of the

family Gramineae (Poaceae). The genus

 

Oryza

contains 25

recognized species, of which 23 are wild species and two,

O.

sativa

and

O.

 

glaberrima

 

are cultivated (Morishima, 1984;

Vaughan

,

 

; Brar and Khush, 2003).

1994

O. sativa

is the

 

most widely grown of the two cultivated species.

 

It is grown

worldwide including in Asian,

 

North and South American,

European

Union,

 

Middle

Eastern

and

African

countries.

However,

 

O. glaberrima

is grown solel

y in West African

 

countries

 

 

Botany of rice

 

pierces its way out through the coleorhizae and the enveloping glume and forms the seminal root. Lateral roots are formed from the seminal root. As the seedlings begin to grow, adventitious roots arise from the basal nodes. When seedlings are pulled out, most of the roots get torn off but very soon fresh roots develop. The root system in rice varies much between the different varieties. Root development is promoted by organic manures and phosphatic manures. Clayey soil seems to be ideal for the normal development of the roots. Nodal roots develop on the higher nodes and help the plant to absorb the food material from the surrounding water.

Culm or Stem The culm, or jointed stem of the rice, is made up of a series of nodes and internodes. The stem of rice popularly known as the haulm or the culm, is generally erect, cylindrical, hollow at the internodes and solid at the nodes. The number and length of internodes vary with the different varieties. There may be ten to twenty internodes. In deep water rice varieties and in

floating types, the culms have generally very long internodes.

Tillers The node is the solid portion of the culm. The node or nodal region bears a leaf and a bud. The bud is attached to the upper portion of the node and is enclosed by the leaf sheath. The bud may give rise to a leaf or a tiller. Early tillers arise from the main culm in an alternate pattern. Primary tillers originate from the lowermost nodes and give rise to secondary tillers. Secondary tillers produce tertiary tillers. Each tiller is an independent plant. The maximum production of tillers is generally attained thirty to forty days after transplanting.

Leaf The node or nodal region of the culm will bear a leaf. Leaves are borne alternately on the culm in opposite directions. One leaf is produced at each node. Varieties differ in the number of leaves produced. The topmost leaf below the panicle is the flag leaf. The flag leaf contributes largely to the filling of grains because it supplies photosynthetic products, mainly to the panicle. The leaf sheath and leaf blade are continuous. A circular collar joins the leaf blade and the leaf sheath. The leaf sheath is wrapped around the culm above the node. The swelling at the base of the leaf sheath,just above the node,is the sheath pulvinus. It is sometimes incorrectly referred to as the node. Leaf blades are generally flat. Varieties differ in blade length, width, thickness, area, shape, color, angle and pubescence.

With many parallel veins on the upper surface of the leaf, the underside of the leaf blade is smooth with a prominent ridge in the middle; the midrib Most leaves possess small, paired ear-like appendages on either side of the base of the blade – called auricles.

Auricles may not be present on older leaves. Another leaf appendage is the ligule, a papery membrane at the inside juncture between the leaf sheath and the blade. It can have either a smooth or hair-like surface. The length, color, and shape of the ligule differ according to variety.

Panicle

The terminal component of the rice tiller is an inflorescence called as panicle. The inflorescence or panicle is borne on the uppermost internode of the culm. The panicle bears rice spikelets, which develop into grains. The panicle base often appears as a hair like ring and is used as a dividing point in measuring culm and panicle length. The panicle base is often called the neck. The panicle axis is continuous and hollow except at the nodes where branches are borne. The swellings at the panicle axis where the branches are borne are referred to as the panicle pulvinus. Each node on the main panicle axis gives rise to primary branches which in turn bears secondary branches. Primary branches may be arranged singly or in pairs. The panicles bear spikelets, most of which develop into grains. These spikelets are borne on the primary and secondary branches. The spikelet is the basic unit of the inflorescence and panicle. It consists of the pedicel and the floret. The floret is borne on the pedicel. The rudimentary glumes are the laterally enlarged, cup like apex of the pedicel. The rudimentary glumes are the lowermost parts of the spikelet. During threshing, the rudimentary glumes are separated from the rest of the spikelet. The sterile lemmas are small, bractlike projections attached to the floret. The rachillae is a small axis that bears the single floret. It is between the sterile lemmas and the floret.

Spikelets

The spikelets are carried on small rachillae at the ends of the branches of the panicle. The spikelet in rice is single flowered enclosed by the lemma and palea. There are six stamens, in two whorls of three each, the filaments being very slender and delicate and having sudden exertion of growth at the time of anthesis. The anthere are linear. The pistil has a single ovary, two styles each with a plumose and laterally exerted stigma ovary, two styles each with a plumose and laterally exerted stigma of different shades of purple. The overy is tricarpellary, single celled, single ovuled and with basal placentation. In most cases the plumose stigmas are dusted with

pollen when they emerge out as the dehiscence of the anthers takes place in many varieties just at the time of opening. Though self-pollination is the rule, natural cross-pollination also occurs, varying from 0.1 to 4.0 percent.

Grain

The ovule after fertilization develops into the seed with its coats completely fused together with the developing ovary wall or pericarp.

Pericarp of fruit coat: The pericarp is made up of distinct layers of quadrangular cells which form the Epicarp. These cells have slight thickening and are followed by cells which are much compressed and form the Mesocarp consisting of two or three layers. The Endocarp is a single layer of tube cells. The color in the rice grain is found in the pericarp layer in the mature stage.

The seed coats: Due to the pressure brought out by the developing seed on the pericarp, the testa and tegmen become much pressed down and out of shape. A few layers of such cells below the pericarp can be diagnosed as the integuments of seed coats.

 

The aleurone layer: A prominent layer of rectangular cells which contain protein lies next to the seed coats. This layer is known as the aleurone layer. The layer in rice is not coloured unlike in case of maize.

The endosperm: the entire mass of tissue below the aleurone layer is made up of cells which contain plenty of starch grains and these form the endosperm.

The embryo: the scutellum has an upper free part which has a fleshy projection known as the ventral scale. Below this upper ventral scale and almost at the middle of the free part there is nother outgrowth which can be called as the inner ventral scale and theis inner ventral scale is peculiar to rice embryos only.

 

THE FAMILY: FABACEAE

Fabaceae family, Leguminosae or fabaceae also called the legume, pea or bean family is the third largest family of the angiosperms after Orchidaceae and Asteraceae and second to Poaceae in terms of agricultural and economic importance. The family comprises of about 751 genera and 19,000 species. Recent molecular and morphological evidence supports the fact that the Fabaceae is a single monophyletic family. The name ‘Fabaceae’ comes from the defunct genus Faba, now included in Vicia. The term “faba” comes from Latin, and appears to simply mean “bean”. Leguminosae is an older name still considered valid, and refers to the fruit of these plants, which are called legumes. All pulses are leguminous but all legumes are not pulses. The species in this family are distributed worldwide but are present in abundance in tropical and subtropical regions. The family is further divided into Papilionaceae , Ceasalpiniaceae and Mimosaceae. A few members of these sub-families :-

1. Papilionaceae – Pea; Cowpea; soybean; lentil; chickpea; groundnut; black gram; green gram; kidney bean; lima bean; pigeon pea; bengal gram; indian bean; cluster bean; alfalfa; fenugreek; broad bean; indigo; flame of the forest; sunhemp; sesbania; shisham; sweet pea; butterfly pea; pongam etc.

2 Cesalpiniaceae – Bauhinia purpurea; gulmohar; tamarind; peacock flower; sita ashoka; Parkinsonia etc.

3. Mimosaceae – Mimosa pudica; Acacia catechu; Ardusi; Acacia nilotica L. Useful plants of Fabaceae

Difference between 3 sub-families

Features Papilionoidae (fabaceae) Caesalpiniaceae Mimoseae Leaves Imparipinnate paripinnate Bipinnate, stipules present or absent Flowers Zygomorphic Slightly Zygomorphic Actinomorphic (Regular) Fruit Legume, small Legume long Lomentum Inflorescence Racemose Racemose Spherical head Calyx Gamosepalous Polysepalous, some times gamosepalous Gamosepalous Corolla Polypetalous, posterior petal largest and outermost, aestivation vexillary Polypetalous, posterior petal smallest and innermost, aestivation imbricate Gamopetalous, all petals equal, aestivation valvate Androecium Stamens 10, (9)+1, rarely (10) 0r 10 10 or fewer Often indefinite, sometimes definate E.g. Pea, Cowpea, green gram Tamarind Acacia nilotica Legume – Papilionoideae flower Standard/Vexillum / Banner Keel/carina Wing/alae Stamens: 9 + 1 Acacia – Mimosoideae Senna – Caesalpinoideae 9

Vegetative characters • Habit : Usually annual or perennial herbs, shrubs, some are tendril climbers like Pisum sativum, Lathyrus odoratus, some are twiners like Clitoria and some are trees like Delbergia sisoo. Root : A much branched tap root system, bearing bacterial nodules. • Stem: Herbaceous or woody, erect or twinner, branched, angular or cylindrical, solid or fistular. • Leaves:

Stipulate, Alternate, Unipinnately compound and imparipinnate, leaf base: Pulvinous ; venation:

Reticulate In Pisum sativum and Lathyrus sativus, upper leaflets are modified into tendrils. Exception: Palmately compound leaves, Ex: Trifolium and Melilotus. 10

Floral characters Flowers: Bracteate, pedicellate, complete, bisexual, pentamerous, dichlamydeous (having two coverings, a calyx and a corolla), zygomorphic and hypogynous. Inflorescence: Racemose Calyx: Sepals usually (5), green, gamosepalous showing valvate or imbricate aestivation. Odd sepal is anterior in position. Corolla: All pulses are leguminous but all legumes are not pulses. The species in this family are distributed worldwide but are. Petals 5, coloured, polypetalous showing descendingly imbricate or vexillary aestivation. The outer most petal is large called standard petal or vexillum or banner. Two lateral petals are lanceolate and curved. They are called wing petals or alae. 11. Two anterior and partly fused innermost petals are called keel petals or carina. The stamens and pistil are enclosed by these keel petals. All the petals have a claw at theirbases. This type of irregular corolla is described as papilionaceous corolla. Sometimes 4. Androecium : Stamens 10, usually diadelphous (stamens divided into 2 bundles). Nine stamens are fused to form a bundle and the tenth stamen is free (9) + 1 (e.g. Clitoria ternatea). The odd stamen is posterior in position. 12. In Crotalaria verrucosa the stamens are monadelphous and dimorphic i.e. 5 stamens have longer filaments and other 5 stamens have shorter filaments. Gynoecium : Ovary superior, monocarpellary, stipulate i.e. ovary has a short stalk at the base. Ovary unilocular with one to many ovules showing marginal placentation. Style simple and bent. Stigma flattened or feathery. Fruit: A legume or pod, splitting along both dorsal and ventral sutures. Seed: Non- endospermic and kidney shaped. Floral formula: Br

% O K (4-5) C 1+2+(2) A (9) + 1 or α G 1 13

 

Cowpea

Cowpea V. unguiculata can grow up to 80 cm and up to 2 m for climbing cultivars. It has a well-developed root system. Germination is epigeal with the first pair of true leaves being simple and opposite and subsequent leaves being trifoliate with oval leaflets (6-15 cm long and 4-11 cm broad) and alternate. The papillonaceous flowers are born on racemose inflorescences at the ends of peduncles that arise from leaf axils and can be white, yellowish, pale blue or violet. Peduncles are stout and grooved and usually much longer than the leaves (2 to 20 cm long). For each inflorescence, flowers are sequentially produced in alternating pairs on thickened nodes at the tip with cushion-like extra-floral nectaries between each pair of flowers. The flower is large (standard is 2-3 cm in diameter), with a straight keel, diadelphous stamens (one free and nine fused), a sessile ovary with many ovules, and a style that is bearded along the inside and ends in an oblique stigma. Pods occur in pairs forming a V, mostly pending and vertical but they can be erect. They are cylindrical, 2 to 6 cm long and 3 to 12 mm broad and contain 8 to 20 seeds. Seeds can be white, pink brown or black (Heuzé et al, 2013)

 

Picture of cowpea flower (top left), immature green pod (top right), maturing pods with an illustration of the great variety of seed colours

(bottom). Source: Courtesy of Carl Davies, CSIRO

 

 

 

 

 

 

 

 

 

 

Soybean

Cultivated soybean (Glycine max) is an annual crop. It belongs to the

family Leguminosae Sub – family Papilionoideae and tribe Phaseolae. It is a diploid with chromosome number 2n=2x=40. Some of the wild and semi-wild relatives e.g. G. falcata and G. latifolia are also diploid. Others such as G. tabacina and G. tomentella are tetraploids (2n = 4x = 80).

Root: Consists of a taproot that may go deep into the soil depending on cultural conditions • The root system is usually in the top 30- 60 cm of soil with about 80-90 of dry weight occurring in the top 15 cm of the soil. Nodules may be visible 10 days after planting if the appropriate rhizobia are present in the soil. The nodulated root system consists of a taproot from which emerges a lateral root system.

Stem Well-developed main stem, may have 0-6 lateral branches and grow to a height of 20 cm to over 100 cm depending on variety and cultural/environmental conditions. The plants of most cultivars are covered with fine trichomes, but glabrous types also exist.

Growth habit of soybean plant. Cultivated soybean is generally an erect, sparsely branched bush type – unlike the spreading sometimes vinery characteristics of wild and semi-wilds. There are two types of stem growth habit and floral initiation in soybean. These are the determinate and indeterminate types. Most of the varieties cultivated in the tropics are determinate while those in the temperate regions are indeterminate.

  1. Determinate: Vegetative growth of terminal bud ceases when it becomes an inflorescence. Flowering occurs about the same time along the length of the plant, hence pod and seed development and maturation occur uniformly throughout the plant. Terminal leaves are of the same size as those at the bottom of the main stem.
  2. Indeterminate: Plant may have achieved less than half its final height when it starts flowering. Plant continues to grow taller while flowering also continue. Flowering/pod and seed development are spread over a longer period. Pod and seed development on lower parts of the plant are more advanced than those at the top. They do not have Terminal leaves are smaller than those which are lower on the plant. The primary leaves are unifoliate, opposite and ovate, the secondary leaves are trifoliolate and alternate, and compound leaves with four or more leaflets are occasionally present.

Leaf: There are two main types: Primary and Trifoliate leaves. The primary leaves are ovate with 1- 2 cm length petioles and a pair of stipules at its point of attachment to the stem • The trifoliate leaves are alternate and have long petioles. Leaflets have entire margins and are broadly ovate to ellipticIanceolate in shape.

Soybean has a typical papilionaceous flower which consists of a tabular calyx with five sepal lobes; a corolla made up of a standard petal, two lateral wing petals, and two anterior keel petals. The stamens form a ring at the base of the stigma and elongate one day before pollination, at which time the elevated anthers form a ring around the stigma. Nine of the ten stamens are fused leaving the posterior one separate. The pistil is made of an ovary that contains 1-5 ovules, a style that accrues towards the standard and a capitate stigma. Pollination occurs before the flower opens. Pods are borne in clusters on short stalks, pubescent and of varied colours depending on cultivar. They may be straight or slightly curved, about 1 cm in width and 3-7 cm long when matured Seeds. Matured soybean seed may be spherical in shape or may be flattened and elongated to various degrees depending on variety. Seed size may range from 5 – 40 g per 100 seeds with most varieties between 10 and 20 g per 100 seeds

 

 

 

 

 

Groundnut

The groundout is a relatively small herb, its height varying between 15 and 60 cm, de- pending on the variety grown. Some varieties grow as a bunch like the bush beans, some are creepers, developing long stems that trail along the ground. Groundnuts have compound leaves consisting of two pairs of little leaves. They develop small yellow flowers. The most peculiar feature of the groundnut is the fact that the fruit develops below the surface of the soil. When the flower has been pollinated, a thin stem grows down towards the soil and penetrates the surface. Only after reaching a depth of 10-15 cm will the fruit start to form. If the thin stem cannot penetrate the soil there will be no fruit. The pods are round and contain between I and 6 seeds. They are ready for harvesting 13 to 20 weeks after planting.

THE FAMILY: EUPHORBIACEAE

Cassava

Root: The roots are the main storage organ in cassava. In plants propagated from true seeds, a typical primary tap root system is developed, similar to dicot species. The radicle of the germinating seed grows vertically downward and develops into a taproot, from which adventitious roots originate. Later, the taproot and some adventitious roots become storage roots. In plants grown from stem cuttings the roots are adventitious and they arise from the basal cut surface of the stake and occasionally from the buds under the soil. These roots develop to make a fibrous root system. Only a few fibrous roots (between three and ten) start to bulk and become storage roots. Most of the other fibrous roots remain thin and continue to function in water and nutrient absorption. Once a fibrous root becomes a storage root, its ability to absorb water and nutrients decrease considerably. The storage roots result from secondary growth of the fibrous roots; thus the soil is penetrated by thin roots, and their enlargement begins only after that

penetration has occurred.

Stem: The mature stem is woody, cylindrical and formed by alternating nodes and internodes. On the nodes of the oldest parts of the stem, there are protuberances, which are the scars left by plant’s first leaves. A plant grown from stem cuttings can produce as many primary stems as there are viable buds on the cutting. In some cultivars with strong apical dominance, only one stem develops.

Cassava plant has sympodial branching. The main stem(s) divide di-, tri-, or tetra-chotomously, producing secondary branches that produce other successive branchings. These branchings, which are induced by flowering, have been called ‘reproductive branchings’.

Leaves: Cassava leaves are simple, formed by the lamina and petiole, the leaf is lobed with palmated veins. There is generally an uneven number of lobes, ranging from three to nine (occasionally 11). Only a few cultivars are characterised by having three-lobed mature vegetative leaves, which may represent the primitive ancestral form (Rogers and Flemming, 1973). Leaves near the inflorescence are generally

reduced in size and lobe number (most frequently three-lobed), but the one closest to the base of the

inflorescence is frequently simple and unlobed. Leaves are alternate and have phyllotaxy of 2/5, indicating that from any leaf (leaf 1) three are two revolutions around the stem to reach the sixth (leaf 6) in the same orthostichy as leaf 1. In these two revolutions there are five successive intermediate leaves (not counting leaf 1). Mature leaves are glaborous and each leaf is surrounded by two stipules approximately 0.5-1.0 cm long), whuch remain attached to the setm when the leaf is completely developed (CIAT, 1984). The petiole length of a fully opened leaf normally varies from 5 to 30 cm, but may reach up to 40 cm.

The upper leaf surface is covered with a shiny, waxy epidermis. Most stomata are located on the lower (abaxial) surf.ace of the leaves; only a few can be found along the main vein on the upper (adaxial) surface (Cerqueira, 1989). Of 1,500 cultivars studied, only 2% had stomata on the adaxial surface (El-Sharkawy and Cock, 1987). The stomata on the upper surface are also functional and bigger than those on the undersurface, Both are morphologically paracytic, with two small guard cells surrounded by two subsidiary cells (Cerqueira, 1989). The number of stomata per leaf area range from 278 to 700 m-2, and all stomatal pores can occupy from 1.4 to 3.1% of the total leaf area.

Flowers: Cassava is a monoecoius plant producing both male (pistillate) and female (staminate) flowers on the same plant. He inflorescence is generally formed at the insertion point of the reproductive branchings; occasionally inflorescences can be found in the axils on the upper part of the plant. The female flowers, located on the lower part of the inflorescence, are fewer in number than male flowers, which are numerous on the upper part of the inflorescence. On the same inflorescence, the female flowers open (1-2 weeks before the male flowers (protogyny), male and female flowers on different branches of the same plant can open at the same time. Normally, cassava is cross pollinated by insects thus it is a highly heterogynous plant.

The flowers do not have a calyx or corolla, but an indefinite structure called perianth or perigonium, made up of yellow, reddish or purple tepals. The male

flower is half the size of the female flower. The pedicel

of the male flower is thin, straight and very short, while that of the female flower is thick, curved and long. Inside the male flower, there is a basal disk divided into ten lobes. Ten stamens originate from between them. They are arranged in two circles and support the anthers. The five external stamens are separated and longer than the inner ones, which join together on the top to form a set of anthers. The pollen is generally yellow or orange, varying from 122 to 148 μm in size, which is very large compared to other flowering plants (Ghosh et al., 1988). The female flower also has a ten-lobed basal disk, which is less lobulated than the male flower. The ovary is tricarpelary with six ridges and is mounted on the basal disk. The three locules contain one ovule each. A very small style is located on top of the ovary, and a stigma with three undulated, fleshy lobes originates from the style.

Fruits and seeds

The fruit is a trilocular capsule, ovoid or globular, 1-1.5 cm in diameter and with six straight, prominent longitudinal ridges or aristae. Each locule contains a single caruncate seed. The fruit has a bicidal dehiscence, whuch is a combination of septicidal and loculicidal dehiscences, with openings along the parallel plane of the dissepiments and along the midveins of the carpels, repectively. With this combination of dehiscences, the fruits open into six valves causing an explosive dehiscence, ejecting the seeds some distance (Rogers, 1965). Fruit maturation generally occurs 75-90 days after pollination (Ghosh et al., 1988). The seeds is ovoid-ellipsoidal, approximately 100 mm long, 6 mm wide and 4 mm thick. The weight varies from 95 to 136 mg per seed (Ghosh et al., 1988). The smooth seed coat is dark brown, mottled with grey. The seeds usually germinate soon after collection, taking about 16 days for germination

 

 

Sweet potato

Growth: The sweet potato is a herbaceous and perennial plant. However, it is grown as an annual plant by vegetative propagation using either storage roots or stem cuttings. Its growth habit is predominantly prostrate with a vine system that expands rapidly horizontally on the ground. The types of growth habit of sweet potatoes are erect, semi-erect, spreading and very spreading

 

 

 

 

 

 

Root: The sweet potato root system consists of fibrous roots that absor nutrients and water, and anchor the plant, and storage roots that are lateral roots which store photosynthetic products. The root systems obtained by vegetative propagation starts with adventitious roots that develop into primary fibrous roots which are branched into lateral roots. As the plant matures, thich pemcil roots that have some lignificaton are produced. Other roots that have no lignification, are fleshy and thiken a lot are called storage roots. Plants grown from true seed form a typical root with a central axie with lateral branches. Later on, the central axie functions as a storage root

Stem: A sweet potato stem is cylindrical and its length, like that of the internodes, depends on the growth habit of the cultivar and of the availability

of water in the soil. The erect cultivars are

Types of roots in sweet potato

approximately 1 m long while the very spreading ones can reach more than 5 m long. Some cultivars have stems with twinning characeristics. The internode length can vary from short to very long and according to stem diameter, can be thin or very thick

 

Parts of the stem

 

Identification criteria for stems include: Amount and location of pigment; Thickness; Hardness; Branching or non-branching habit; Stem emergence; Internode length (between leaves); Node size (swollen nodes are typical of a few varieties); Wing structure either wavy or straight. Wings are protruding ridges running along the stem. Their characteristics of being wavy or straight are often very useful in confirming variety identification In most potato variety descriptions, mention is only made to wing structure when it is wavy. In all other cases it is straight or nearly so. Incidentally, most varieties have hollow internodes.

Leaves: A typical potato leaf is composed of two to four pairs of primary leaflets arranged on the midrib with a terminal leaflet on the end. Between the primary leaflets are smaller ones, called secondary leaflets, which are often placed irregularly along the mid-rib. Tiny inconspicuous rudimentary or tertiary leaflets can also be evident along the mid-rib. The leaf is attached to the plant by a petiole. Depending on the variety, the leaf may: be long or short; be rigid, drooping or spreading; be flat or with side leaflets arched or drooping; or have a distinct petiole angle between leaf and stem which occasionally can be used as a distinguishing feature. The edge of the leaf can be entire, toothed or lobed. The base of the leaf lamina generally has two lobes that can be almost straight or rounded.

The shape of the general outline of sweet potato leaves can be rounded, reinformed (kidney shaped), cordate (heart shaped), triangular, hastate (triobular or spear-shaped with the two basal lobes divergent). The number of lobes generally range from 3 to 7 and can be easily determined by counting the veins that go from the junction of the petiole up to the edge of the leaf lamina. However, toothed leaves have minute lobes called teeth which could number from 1 to more than 9. Some cultivars show some variations in leaf shape

 

General outline of the leaf

 

Inflorescence

Sweet potato generally differs in their ability to flower. Under normal conditions in the field, some cultivars do not flower, others produce very few flowers and others flower profusely. The inflorescence is generally a cyme in which the peduncle is divided in two axillary peduncles; each is further divided in two after the flower is produced (biparous cyme). In general, buds of first, second and third order are developed. However, single flowers are also formed. The flower buds are joined to the peduncle through a very short stalk called pedicel. The colour of the flower bud, pedicel and pedunle varies from green to completely purple pigmented. The flower is bisexual. Besides the calyx and corolla, they contain the stamens that are the male organs or androecium and the pistil that is the female organ or gynoecium. The calyx consists of sepals, 2 outer and 3 inner, that stay attached to the floral axile after the petals dry up and fall. The corolla consists of 5 petals, that are fised forming a funnel, generally with lilac or pale purple limb and with reddish to purple throat (the inside of the tube). Some cultivars produce white flowers. The androecium consists of five stamens with filaments that are that are covered with glandular hairs and that are partly fused to the corolla. The length of the filaments is variable in relation to the position of the stigma. The anthesis are whitish, yellow or pink, with a longitudinal dehiscence. The pollen grains are spherical with the surface covered with very small glandular hairs. The gynoecium consists of a pistil with a superior ovary, two carpels and two locules that contain one or two ovules. The style is relatively short and ends in a broad stigma that is divided into two lobes that are covered with glandular hairs. At the base of the ovary, there are basal yellow glands that contain insect-attracting nectar. The stigma is receptive early in the morning and the pollination is mainly by bees.

 

 

 

Types of different lobes

 

Parts of the flower

 

 

 

 

Parts of the flower

Fruits and seeds

The fruit is a capsule or less spherical with a terminal tip, and can be pubescent or glaborous. The capsule turns brown when mature. Each capsule contains from one to four seeds that are slightly flattened on one side and convex on the other. Seed shape can be irregular, slightly angular or rounded; the colour ranges from brown to black; and the size is approximately 3 mm. the embryo and endo sperm are protected by a thic, very hard and impermeable teasa. Seed germination is difficult and requires scarifificstion by mechanical abrasion or chemical treatment. Sweet potato seeds do not have a dormancy period but maintain their viability for many years.

 

The fruit is a capsule

with one to four seeds

 

Storage root

The storage roots are the commercial part of the sweet potato plant, and sometimes are mistakenly named ‘tubers’. Most cultivars develop storage roots at the nodes of the mother stem cuttings that are underground. However, the very spreading cultivars produce roots at some of the nodes that come into contact with the soil. The parts of the storage roots are the proximal end that joins to the stem, through a root stalk, and where many adventitious buds are found from which the sprouts are originated; a central part which is more expanded, and the distal end that is opposite

to the root stalk. The adventitious buds that are located in the central and distal part usually sprout later than those located in the proximal end

THE FAMILY: PALMAE ARECACEAE

The palm family Arecaceae (formerly known as the Palmae) is placed in the order Arecales in the monocotyledons (Cronquist, 1981). The oil palm Elaeis guineensis Jacq. is grouped with Cocos (the coconut) and other genera in the subfamily Arecoideae and tribe Cocoseae (Dransfield et al., 2005). There are currently two accepted species of Elaeis, E. guineensis and Elaeis oleifera, the African and American oil palms.

The botany of Oil palm

E. guineensis is a large, pinnate‐leaved palm having a solitary columnar stem with short internodes. There are short spines on the leaf petiole and within the fruit bunch. The separate upper and lower ranks of leaflets on the rachis give the palm a characteristic untidy appearance. The palm is normally monoecious with male or female, but sometimes mixed, inflorescences developing in the axils of the leaves. The fruits are borne in a large, compact bunch. The fruit pulp, which provides palm oil, surrounds a nut, the shell of which encloses the palm kernel. The description by Jacquin (1763) was detailed, but he described the flowers as either female or hermaphrodite steriles and seemed unaware that flowers of the two sexes were in separate inflorescences. Gaertner (Defructibus et seminibus plantarum, Stuttgart, 1788) gave a more detailed description of the flower parts, recording that the male and female flowers are on separate inflorescences.

 

 

Janssens (1927) and Smith (1935) provided the first simple classifications of the fruit, which have

The oil palm tree and its reproductive parts stood the test of time. The important terms are ‘fruit type’ to describe the external appearance of the fruit and ‘fruit form’ for internal anatomy.

Janssens recognised that the fruit forms dura and tenera, distinguished by the thickness of shell, could be found in fruit types of different external appearance. Thus, both the common fruit type nigrescens and the green‐fruited virescens were divided by Janssens into three forms, dura, tenera and shell‐less pisifera (but today, these are often confusingly called fruit types). The pale‐fruited albescens was also recognised, but only a dura albescens had been found. Smith recognised both mantled (Poissoni) and unmantled nigrescens and virescens fruit, called them ‘types’, and divided all four into thick‐shelled and thin‐shelled ‘forms’. The term variety is inappropriate for the tenera and other forms, either in the botanical or in the plant breeding sense, as material will be heterogeneous for most characters other than shell thickness. Some authors, and seed producers, still persist in using the term variety, however.

The oil palm nut consists of a shell, or endocarp, and in the majority of cases a single kernel, since two of the three ovules in the tricarpellate ovary usually abort. Abnormal ovaries sometimes occur, and four‐ or five‐ seeded nuts may, very rarely, arise from these. In botanical terms, the kernel is the

seed, but the word ‘seed’ is commonly used for the nut, since it is the nut that is stored, germinated and planted. Nut size varies greatly and depends on both the thickness of the shell and the size of the kernel. Typical African dura nuts may be 2–3 cm in length and average 4–5 g in weight, although some are larger. Deli dura nuts average 5–6 g and range up to 13 g. Tenera nuts are usually 2 cm or less in length and average 2 g, but nuts weighing 1 g are not uncommon (Plate IID). The shell has fibres passing longitudinally through it and adhering to it, and drawn into a tuft at the base. There are three germ pores corresponding to the three parts of the tricarpellate ovary, but the number of functional pores corresponds to the number of kernels developed. A plug of fibre is formed in each germ pore, and these fibres are cemented together at the base to form a plate‐like structure continuous with the inner surface of the shell (Hussey, 1958). Inside the shell lies the kernel. This consists of layers of hard oily endosperm, greyish white in colour, surrounded by a dark‐brown testa covered with a network of fibres. Embedded in the endosperm and opposite one of the germ pores lies the embryo, about 3 mm in length, with its distal end opposite the germ pore but separated from it by a thin layer of endosperm cells, the testa and the plate‐like structure referred to above. These three structures have been together called the operculum, but they are separate.

The process of seed germination involves that the emerging embryo forms a ‘button’ [commonly called the hypocotyl but considered by Henry (1951) to represent the petiole of the cotyledon]. The plumule (seedling shoot) and radicle both emerge through a cylindrical, persistent ligule close to the seed. Inside the seed, the haustorium develops steadily. This organ is yellowish and convoluted along the long axis of the nut, thus providing a large surface area for absorption. Within about 3 months after germination starts, the spongy haustorium has absorbed the endosperm and completely fills the nut cavity (Anon., 1956)

Fig. The oil palm seed and early growth of seedling (a) longitudinal section of seed through embryo; (b) just germinated seed; (c) median longitudinal section of embryo; (d-g) successive stages in the early growth of the embryo; (h) production of adventitious roots; (i) 4week-old seedling; (j) section of seed to show hastorium; r: radicle; r’:

adventitious root; s: shell; I-III plumular leaves (From Rees (1960)

with permission from the Editor, Palms (formerly Principes).)

The leaf:

Development of the leaf is initially very slow. There are some 40–60 leaves within the apical bud; each remains enclosed for about 2 years, then rapidly develops into a central ‘spear’ and finally opens (Henry, 1955a, c; Broekmans, 1957b). The base of the developing leaf completely encircles the stem apex, and in the adult leaf, this leaf sheath persists as a strong, fibrous sheet.

The mature leaf is simply pinnate, bearing linear leaflets or pinnae on each side of the leaf stalk. The latter is divided into two zones: the rachis bearing the leaflets and the petiole, which is shorter than the rachis, bearing only short lateral spines (Anon., 1962). At the junction of petiole and rachis, small leaflets with vestigial laminae are found. Petioles vary greatly in length and may be as long as 1.2m. The lower (abaxial) surface of the petiole is green, yellowish green or yellow ochre, sometimes with a darker central stripe and often with a whitish area at the base. This variation in colouring appears to be genetic and can be helpful in distinguishing between different clones. The upper surface of the petiole is greyish. The rachis is similar in colour to the lower surface of the petiole.

The root system

In the mature palm, thousands of primary roots spread from the bole, with new primaries continually replacing dead ones (Yampolsky, 1922). Roots sometimes develop on the stem up to 1 m above ground; these normally stop growing and dry out before reaching the soil but may continue to grow if soil is heaped around the base of the stem. The extent of the root system depends on soil characteristics The general pattern has been known for a long time (von Mohl, 1849; Purvis, 1956), and researchers have agreed that there are four main classes of roots. There are no reports of root hairs. The primary roots are 6–10 mm in diameter; these carry secondary roots (2–4 mm), which carry branched tertiaries (0.7–1.2 mm) and then quaternaries (0.1–0.3 mm diameter and 1–4 mm long) (Tinker, 1976). There is no taproot, but primary roots, 5–10 mm in diameter, extend either downwards from the base of the palm or outwards in a more or less horizontal direction (Fig. 2.5). Most authors indicate a distinction between vertical and horizontal roots, with little in between (e.g. Ruer, 1967b). The descending primaries, which proceed directly from under the base of the palm, are fewer in number than the surface primaries and carry fewer secondaries. Ruer (1969) showed that the descending roots played some part in the absorption of water: when they were cut, stomatal opening was significantly reduced. If all primary roots except for the deep roots were cut, however, the stomata remained almost completely closed, so the surface roots appeared more important for water uptake

Inflorescences

Male inflorescence

 

Female inflorescence

 

The oil palm is monoecious; that is, male and female flowers occur separately on the same plant, usually in distinct male and female inflorescences thus minimising the chance of self‐pollination. Detailed investigation of the flowers has shown, however, that each flower primordium includes both male and female organs (Beirnaert, 1935b). In rare cases, both develop fully to give a hermaphrodite flower. An inflorescence is initiated in the axil of every leaf but some inflorescences abort before emergence. Rarely, twin inflorescences are found in a single leaf axil. The first inflorescences produced by young palms are usually male, but thereafter, the order and proportions in which inflorescences are produced show little or no regularity. Mixed

inflorescences, with both male and female spikelets, are more common in young palms. Each inflorescence consists of a stout peduncle 30–45 cm in length, with spikelets arranged spirally around it. Phyllotaxis measurements have shown little difference in spikelet arrangement between male and female inflorescences (Thomas et al., 1970). For most of its development, the inflorescence is completely enclosed in two fibrous spathes. About 6 weeks before anthesis, the outer spathe splits; after a further 2 or 3 weeks, the inner spathe also splits, and

later, both spathes fray and disintegrate, exposing the inflorescence. Female inflorescence while still on the tree

The fruit is a sessile drupe varying in shape from nearly spherical to ovoid or elongated and bulging somewhat at the top. In length, it varies from about 2 cm to more than 5 cm, with an average weight of about 10 g. The pericarp of the fruit consists of the outer exocarp or skin, the mesocarp or pulp (often incorrectly termed the pericarp) and the endocarp or shell. The endocarp together with the kernel forms the seed, described previously.

 

THE FAMILY: MALVACEAE

Cocoa plant- Theobroma cacao

Cocoa (Theobroma cacao L.), known worldwide for being the raw material of chocolate, belongs to the class Magnoliopsida, order Malvales, family Malvaceae, genus Theobroma and species Cacao, being the main fruit of the genus cultivated, due to the value and importance of the seeds.

Cultivars

Cultivars fall into three categories, one of two

botanical varieties or their hybrids: Cocoa immature pod

  1. Criollo (T. cacao var. cacao). The word criollo means “native”, as it is distributed from southern Mexico to South America, north and west of the Andes. Fruits are oblong to ovoid in shape, tapering to a point, and have five or ten longitudinal ridges; seeds have yellowish white cotyledons.
  2. Forastero (T. cacao var. sphaerocarpum). The word forastero means “foreign”, as it was introduced to Mesoamerica from the Amazon basin. The fruits are ellipsoid to round, lacking a pointed tip, and may be furrowed but have a smooth surface otherwise. The cotyledons are violet. Forasteros are higher yielding and more vigorous than criollos, but considered to have inferior quality. About 80 to 90 percent of cacao production is based on the forastero form, due to its superior yield, vigor, and disease resistance. ‘Amelonado’ is the major West African cultivar, and the predominant type grown worldwide.
  3. Trinitario (hybrids of Criollo and Forastero forms). These hybrids, which originated in Trinidad, are sometimes classified as a subgroup of the forasteros. Since they are hybrids, they are highly variable from seed, unless the seed is derived from known crosses. The seed quality is intermediate between that of the criollos and the forasteros, as are other characteristics.

Root system: consists of a pivoting root that has its length and shape varying according to the structure, texture, and consistency of the soil. In deep soils with good aeration it can have a growth of the pivoting root of up to 2 m. The secondary roots are responsible for the plant nutrition, and generally 70%90% of these are in the first 30 cm of the soil.

Stem: it is erect, and with 2-years aged, the growth of the terminal yolk is stopped with 1.01.5 m of height. Afterwards, the first crowns appearing, composed of 35 main branches, that multiply in other lateral and secondary branches. In the first years, the cacao tree presents smooth stems bark. Later, due to development of flowers cushions, it becomes rough and rugged.

Leaves: the leaves are oblong, acuminate, and glabas with prominent central rib. When new, depending on the clone or cultivar, they have a color ranging from green (more or less rosy) to violet, depending on the amount of anthocyanin present. When old, the leaves lose their pigmentation, becoming pale green, and

finally, dark green and stiff.

Flowers: Cacao flowers appear in floral cushions on the trunk or woody branches, from buds that develop in the armpits of old leaves. The flowers are hermaphroditic and have the following constitution: five sepals, five petals, five estaminodes, five stamens and one pistil whose ovary has five ovules. The cacao flowers have structural characteristics that limit their pollination exclusively by insects. The main pollinating agents of cacao are a small group of insects belonging to the Ceratopogonidae family, genus Forcipomya. In the Amazon Region, the cacao tree has two flowering peaks: a minor that coincides with the beginning of the less rainy period and a main one that occurs at the end of the dry season and the beginning of the rainy season. Annually, an adult cacao tree can produce more than 100,000 flowers, but only about 0.1% turn into fruit. The unpollinated flowers fall within 48 h. On the other hand, the pollinated and fertilized flowers remain fixed on the peduncle, and they develop the fruit.

Fruit: it presents a fleshy pericarp composed of three distinct parts: the epicarp, which is fleshy and thick, whose outer epidermal extract may be pigmented. The mesocarp, which is thin and hard, but not very lignified, and the endocarp, which is fleshy and not very thick. Usually the fruit when immature is green, and yellow when ripe. Others are purple (red-wine) in the development phase and orange in the ripening period. The period between pollination and fruit ripening varies from 140 to 205 days, with an average of 167 days. The fruit index (number of fruits required to obtain 1 kg of commercial cocoa) is generally from 15 to 31 fruits Mature pod of cocoa on the stem ready for harvesting

C

ocoa leaf

 

 

 

 

 

 

 

 

 

 

 

 

Cocoa flower

Seed: the shape varies from ellipsoid to ovoid with 23 cm in length. It is covered by white mucilaginous pulp that has an acid-sweet taste. The embryo has two cotyledons with colors ranging from white to violet. Cocoa’s seeds are very sensitive to temperature changes and die in a short time when suffer from dehydration

 

 

Cocoa pod cut transversely to expose the seed

 

 

THE END OF CRP 201 LECTURE NOTE 2

 

 

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