CS 153: BOTANY OF CROP PLANTS
Course Outline
 Introduction
 Branches of Botany
 Plant Classification and Nomenclature
 Plant Structure
 Seed Morphology
 Root: Functions, Internal Structure, Types, Modifications
 Stem: Functions, Types, Internal Structure, Modifications
 Leaf: Types, Functions, Internal Structure, Leaf Venation, Phyllotaxy
 Reproduction in Plants
 Flower: Types and structure, Pollination and Fertilization
 Fruit Development and Types
 Dispersal of seeds and Fruits
References
1. Botany (An Introduction to Plant Biology) 5Ed, James D, Mauseth
2. Botany for Degree Students, A. C. Dutta (Revised Edition)
3. Principles of Botany. Gordon Uno, Richard Storey, Randy Moore
4. others
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CS 153: BOTANY OF CROP PLANTS
Introduction
The science that deals with living objects goes by the general name of biology (bios, life; logos,
discourse or science). Since both plants and animals are living, biology includes a study of both.
Biology is, therefore, divided into two branches: botany (botane, herb) which deals with plants
and zoology (zoon, animal) which deals with animals.
Scope of botany
Botany deals with the study of plants from many points of view. This science investigates the
internal and external structures of plants, their functions in regard to nutrition, growth, movement
and reproduction, their adaptations to varying conditions of the environment, their distribution in
space and time, their life history, relationships and classification, the laws of heredity, the uses that
plants may be put to and, lastly, the different methods that can be adopted to improve plants for
the use of mankind.
What is Botany? Provide the definition as taught in class.
Branches of Botany
1. Morphology (morphe, form; logos, study): The study of the external form of the plant and
how it attains it.
2.
Anatomy: The study of the gross internal structure of a plant organ and how it comes about.
3.
Histology (histos, tissue): The study of the detailed structure of tissues making up a
particular organ of the plant.
4.
Cytology (kytos, cell): The study of individual cells and their contents
5.
Genetics: The study of heredity which is transmission of the characteristics of individuals
from one generation to another.
6.
Plant breeding: The use of genetic principles for the improvement of plants.
7.
Physiology (physis, nature of life): This is the study of the various functions that plants
perform. This includes all the living processes that go on in the leaf, stem, roots and other
organs e.g. Food manufacturing, transport and storage, absorption of water and nutrients,
transpiration.
8.
Ecology (oikos, home): The study of the inter-relationships between plants and their
environment.
9.
Plant Geography: This deals with the distribution of plants over the surface of the earth and
the factors responsible for this distribution.
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10. Taxonomy or Systematic Botany: This deals with the description and identification of
plants, and their classification into various natural groups according to the similarities and
differences between their morphological characteristics.
11. Agronomy: Is the branch of botany which emphasizes the application of botany into farm
practices. It is concern with the culture, harvest and improvement of field crop. It includes
plant breeding, nutrition, the maintenance of soil fertility and crop protection.
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PLANT CLASSIFICATION
The Binomial System of Nomenclature
The scientific name of every plant consist of two parts the first part designate genus and second
species. This is what is called the binomial system. It was first proposed by the Swedish nationalist
Carl Linnaeus (1707-1778).
The specie is regarded as a group of closely related individuals capable of breeding successfully
with each other but not with other different groups. Species that are different but related by descent
are group together in the genus.
Taxon order from high to the lowest (downwards)
Kingdom
Sub-kingdom
Division / Phyla
Sub-division
Class
Sub-class
Order
Family
Genus
Specie
The plant kingdom consists of over 300,000 different kinds of plants and there are various schemes
of classifications.
A tentative scheme based on modern natural classifications, satisfying our needs, is as follows:
Kingdom Plantae
Sub-kingdom A. Thallophyta (plants not forming embryos)
Division I. Phycophyta or Algae
Sub-division: Cyanophyta or blue-green algae
Euglenophyta or euglenoids
Bacillariophyta or diatoms
Chlorophyta or green algae
Phaeophyta or brown algae
Rhodophyta or red algae
Division II. Fungi or Mycophyta
Sub-division: Schizomycophyta or bacteria
Myxomycophyta or slime fungi
Eumycophyta or true fungi;
Sub-division: Eumycophyta
Class: Phycomycetes or alga-like fungi,
Ascomycetes or sac fungi
Basidiomycetes or club fungi
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Sub-kingdom B. Embryophyta (plants forming embryos)
Division I. Bryophyta (plants without vascular tissues, they are valuable in soil formation)
Sub-division: Hepaticae or liverworts
Anthocerotae or horned liverworts
Musci or mosses
Division II. Tracheophyta (includes a large number of vascular plants and of economic
importance)
Sub-division: Psilotopsida (extinct)
Lycopsida (club mosses)
Sphenopsida (horsetails)
Pteropsida (ferns and seed bearing plants)
Sub-division: Pteropsida
Class: Filicinae (Ferns)
Gymnospermae (coniferous trees)
Angiospermae (flowering plants)
Class: Angiospermae
Sub-class: (i) Monocotyledonae (monocots/ grasses)
(ii) Dicotyledonae (dicots/ broad leaves)
E.g. Classification of a monot and dicot
Taxon
Maize
Groundnut
Kingdom
Plantae
Plantae
Division
Tracheophyta
Tracheophyta
Sub-division
Pteropsida
Pteropsida
Class
Angiospermae
Angiospermae
Sub-class
Monocotyledonae
Dicotyledonae
Order
Graminales
Leguminosae
Family
Gramineae
Papilionaceae
Genus
Zea
Arachis
Specie
mays
hypogaea
Note: underline the scientific names when writing or italize when typing
Varieties (Cultivars)
A variety is a group of rather closely related plant lower than a species. They are cultivated and
recognized agriculturally as a unit. The quality of the products produced by such a group of plants
e.g. nutritional difference and their yields are important characteristics of a variety that go beyond
the botanical standards that are used to determine general species etc.
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THE PLANT STRUCTURE
The plant kingdom is characterized by great diversity and form. Bacteria are microscopic lower
forms of plant life, may be unicellular bodies or cells may be joined to take the form of a slender
thread, or may be grouped into a colony or organized into a primitive structure called thallus. The
higher forms are elaborate and complex. They have roots, stems and leaves. They may be tall and
slender or short and bushy or trailing in growth with stems spreading over the soil surface or
twining about erect plants. There are succulent or the herbaceous types whose stems die-back each
season and woody types such as crops which continue to grow from year to year.
Inspite of this diversity in the higher plant, a simple pattern of development occurs
Fig. 1
The plant body consists fundamentally of axis which at one end becomes the root and at the other
the shoot. The shoots intend is made up of leaves. The root serves to anchor the plant and to absorb
water and minerals from the soil. A young stem (twig) is marked by the presence of nodes which
are the point on the stem where a leaf or leaves are attached. The intervals between the nodes are
called internodes. Axillary buds are usually found in the angle between the leaf and the stem. The
buds may grow into branches that duplicate the structure of the shoot. The leaves are arranged on
the stem in a manner that is constant for any given specie.
The root bears no appendages comparable to the leaves and therefore has no nodes or internodes.
The lateral roots may be regarded as branches of the root. But unlike the shoot the branches arise
within the tissues of the parent roots rather than externally visible buds.
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CLASSIFICATION OF PLANTS BASED ON LONGEVITY OF AERIAL PARTS
They may be classified according to length of life of the entire plant or sometimes only of its aerial
portion.
1. Trees are large plants with a single stout trunk and hard, and woody branches profusely
formed (except most palms).
2.
Shrubs are medium-sized plants with hard and woody stems which branch profusely from
near the ground so that the plants often become bushy in habit without having a clear trunk.
Trees and shrubs have shoots that live for a number of years.
3.
Plants in which the aerial portion is relatively short lived are called herbs or herbaceous
plants. Based on the duration of their life they may be classified as
a.
Annuals: plants that attain their full growth in one season, living for a few months or at
most for one year producing flowers, fruits and seeds within this period, e.g. rice, pea
b.
Biennials: plants that live for two years. They attain their full vegetative growth in the first
year and produce flowers and seeds in the second year, after which they die off e.g. radish,
carrot etc. (In tropical climates they behave like annuals.)
c.
Perennials are those plants that persist for a number of years. The aerial parts of such plants
may die down every year at the end of the flowering season, but next year new shoots
develop again from the underground stem after a few showers, e.g. ginger.
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SEED MORPHOLOGY
Seed
The seed is a complex structure composed of an embryo, seed coat and a supply of stored food.
The most important of them is the embryo.
Embryo is a very young plant and consist of a primary axis with the following component parts
root, hypocotyls and stem, at this stage it is difficult to differentiate where the root joins the
hypocotyls, but above the hypocotyls is the node of the first two leaves i.e. cotyledons referred to
as seed leaves and it marks the beginning of the stem. The components remain quiescent until the
seed germinates.
Types of Seeds
a. Endospermous Seed: The endosperm is the nutritive tissue on which the developing
embryo feeds. In some seeds including almost all monocots and many dicots, the endosperm
persist and serves as food for the embryo after germination, these are endospermous seed.
Examples are castor bean, rice etc.
b.
Non-endospermous seeds are those in which the nutritive tissue is completely absorb before
the seed is ripe. Examples are bean, cucurbita etc.
Fig. 2
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Monocotyledons and Dicotyledons Seeds
Monocot produce seeds each with only one cotyledon, or seed leaf. The cotyledon is a modified
leaf involved in storing or supplying nutrients and energy for the embryo in the seed. Dicots
produce seeds with two cotyledons. When the development of seed is complete it usually dries out
and enters into a period of dormancy.
GERMINATION: Is the process by which the dormant embryo wakes up, grows out of the seedcoat and establishes itself as a seedling. The embryo grows by absorbing food material stored up
in the cotyledons, or in the endosperm when it is present. There are two kinds of germination,
epigeal and hypogeal.
GERMINATION OF THE BEAN SEED
When the seed of bean is placed under favourable condition of moisture, temperature, and air,
various parts of the embryo begin to grow. First, the root appears and grows downward into the
substrate or soil. It is described as been positively geotrophic. Root hairs which are very small and
delicate extensions of the epidermal cells from the back of the root cap absorb water and nutrients
and give the plant its first anchorage.
Role of hypocotyl in germination
As the radicle grows downwards the hypocotyl begins to grow upwards. Since the two large
cotyledons and the stems are above the hypocotyl, they are pulled upwards by the curved hypocotyl
as it elongates. The curved or arched growth of the hypocotyl prevents the breaking of the
cotyledon as it moves up through the soil and becomes exposed to light, further elongation
diminishes rapidly except in the arch itself. The underside with much less exposure to light
continues to grow while the growth of the upper and exposed side is retarded with much differential
growth. The hypocotyl straightens and simultaneously pulls the cotyledon out of the soil. The
cotyledon spreads apart above the soil as two fleshy leaves. They acquire green chlorophyll rapidly
and manufacture food until several other leaves above them develop then the cotyledon shrivel
and drop off the stem this is referred to as epigeal.
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Fig. 3
Hypogeal Germination
In most monocots e.g. maize and some dicots, the hypocotyl does not elongate and the cotyledons
remain below the surface of the soil. The plumule produces a stem that arches until it reaches the
soil surface then it straightens and grows upwards. The cotyledons in the soil finally disintegrate
after the redrawal of food by the seedling e.g. garden pea (Pisum sativum)
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SEEDING DEVELOPMENT OF MAIZE (MAIZE CARYOSIS)
Fig. 4
The cotyledon in maize has undergone extreme evolutionary modification. It is composed of two
main parts the scutellum which is food absorbing organ and the coleoptiles which is a protective
cap over the plumule. During germination the first organ to emerge is the radicle which breaks
through the coleorhiza and then through the tip of the seed. Very soon seminal roots appear at the
scutellar node i.e. junction in the embryo between the radicle and the plumule.
The primary root system formed from the radicle never becomes large and may be temporally.
These primary roots are supplemented by a stronger secondary root system adventitious in origin
which develops from the lower nodes of the stem.
Emergence of the corn seedling is brought about by elongation of the mesocotyl. The mesocotyl
is regarded as a combination of the hypocotyls and of the cotyledonary tissues that originally
connected to the scutellum and the coleoptiles. The elongation of the mesocotyl depends on a
supply of hormones. This hormone supply is adequate to stimulate the growth of the plumule and
the secondary roots.
When the tip of the coleoptiles breaks through the soil surface, rate of auxin production is sharply
reduced by absorption of light. The growth processes are then reversed. Elongation of the
mesocotyl ceases. The plumule emerges from the coleoptiles and roots develop from the first node.
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THE ROOT
The root is the descending portion of the plant and is not normally green in colour, ends in a
protective cap known as root cap, bears unicellular hairs and has no nodes or internodes.
The root performs four key functions for the plant.
A) It is the site for absorption of water and nutrients.
B) The root anchors the plants and gives it stability
C) The root serves as a translocation system for absorbed water and nutrients
D) In some plants the roots serve as a site of storage for accumulated carbohydrate.
ROOT STRUCTURE
The root tip which is one of the plants epical meristem is covered and protected by a thimbleshaped structure called Root cap. New root cap cells are formed constantly in the inner of the cap.
Cells of the root cap can secrete large amounts of mucigel, a slimy substance containing sugars,
enzymes and amino acids that lubricates roots as they push between soil particles. Mucigel also
protects roots from drying out, makes it possible for minerals, along with water in the soil, to be
better absorbed by roots. Due to the impact of the hard soil particles, the outer part of the root cap
wears away and newer cells formed by the underlying growing tissue are added to it.
Zone of Cell Division: This is the growing apex of the root lying within and a little beyond the
root-cap and extends to a length of one to a few millimeters. It consists mainly of the apical
meristem of the root, where new root cells are produced. These cells divide every 12 to 36 hours;
in some plants the meristem produces almost 20,000 new cells each day. Some of the newly formed
cells contribute to the formation of the root-cap and others to the next upper region.
Zone of Cell Elongation: This lies above the meristematic region and extends to a length of a few
millimeters (1 to 5mm, or a little more). The cells of this region undergo rapid elongation
(primarily by filling their vacuoles with water) and enlargement, and are responsible for the growth
in the length of the root by shoving the root-cap and apical meristem through the soil.
Zone of Maturation: This region lies above the region of elongation and extends upwards.
Externally, often extending to a length of a few millimeters and sometimes a few centimeters, this
region produces a cluster of very fine and delicate thread-like structures known as root-hairs. Root
hairs provide a very efficient contact with the soil for absorption of water and nutrients. Internally,
the cells of this zone are seen to undergo maturation and differentiation into various kinds of
primary tissues.
Fig. 5 Root Structure
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Primary tissues: The root apex (apical meristem) is a region of tissues initiation, it gives rise to the
primary meristem i.e. the protoderm, ground meristem and procambium.
Fig. 6 Internal Structure of the Root
A cross section through a mature dicot root gives us a look at its primary structure. A typical dicot
root is surrounded by a layer of epidermis cells. The root epidermis usually either lacks a cuticle
(a thin coating of wax) or has a thin cuticle that does not significantly affect water absorption.
Xylem and phloem cells are packed together in the center of the root. Cortex cells constitute the
majority of cells in the root and often store most of the starch. In many plants, the outmost layer(s)
of the cortex is a protective layer called the hypodermis, with cells lined with suberin, a waxy
substance that is impervious to water.
Fig. 7
Most of the cortex consists of thin-walled, starch-storing parenchyma cells separated by large
intercellular spaces that can occupy as much as 30% of the root’s volume. The innermost layer of
the cortex is the endodermis. Cells of the endodermis are packed tightly together and lack
intercellular spaces. Furthermore, four of the six sides of each endodermis cells are impregnated
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with a Casparian strip made of lignin and suberin and arranged similarly to a rubber band around
a rectangular box. If endodermis cells are likened to bricks in a brick wall, then the Casparian strip
is analogous to the mortar surrounding each brick. The Casparian strip redirects the inward
movement of water and nutrients as they flow through the endodermis; water and dissolved
minerals must pass through the cell membranes of endodermis cells to reach the vascular tissues
in the center of the root, the stele.
Fig. 8
The stele of a root includes all of the tissues in the middle of the root and consists of the pericycle,
vascular tissues (xylem & phloem), and sometimes a pith. The outermost layer of the stele is the
pericycle, a layer of thin-walled meristem cells one to several cells thick. It’s important because
it produces branch roots. The earliest sign of branch-root formation is cell divisions in the
pericycle. Soon thereafter, a root cap and primary tissues form. The branch root then forces its way
through the soil. The vascular tissues of branch roots link with those of the parent root.
Fig. 9
Inside the pericycle is the root’s vascular tissue. Roots of most dicots and some monocots (e.g.
wheat and barley) have a lobed, solid core of xylem in the center. Phloem is found between the
lobes of xylem. The roots of many monocots and few dicots have a ring of vascular tissue
surrounding pith made of parenchyma cells.
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Fig. 10
Note arrangement of vascular bundles in root as compared to the stem.
Note the arrangement of xylem to the phloem (i.e. alternate arrangement instead of all being
on the same radius)
TYPES OF ROOTS
The primary root and its branches form the tap root system of the plant. The primary or tap root
normally grows vertically downwards to a shorter or longer depth, while the branched roots
(secondary, tertiary, etc.) grow obliquely downwards, or in many cases spread horizontally
outwards. This is common in cone-bearing trees and dicots.
When a primary root is not dominant in its growth and it is superseded by the growth of its branches
or fine roots arising out of the lower nodes of its stem, the plant has a fibrous root system as occurs
in most monocots (including grasses).
Fig. 11
ADVENTITIOUS ROOTS
Roots that grow from any part of the plant body other than the radicle are called adventitious
roots. They may develop from the base of the stem, replacing the primary root or in addition to it,
or from any node or internode of the stem or the branch, or even from the leaf, under special
circumstances. Fibrous and Foliar Roots are forms of adventitious roots.
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MODIFIED ROOTS
Aerial Roots: these are adventitious roots because they grow from the stem. They serve several
functions including the following, some are photosynthetic, some are breathing roots
(pneumatophores) these roots have lenticels in the outer tissues for gaseous exchange, aerial roots
for additional support e.g. prop roots in maize, climbing roots in betel (Piper betle), buttress roots
in kapok (Ceiba).
Storage roots: roots of most plants act as storage organs without being modified in any way. In
carrot (Darcus carota) and beet root (Beta vulgaris) the tap root is modified by being greatly
swollen. In both plants the storage organ starts to form at a very early stage and the top part is the
swollen hypocotyl.
Root tubers: it’s a swollen lateral or adventitious root. The edible tuber of cassava (Manihot
esculenta) and sweet potatoes (Ipomoea batatas) are both swollen adventitious roots.
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THE STEM
Characteristics of the Stem
The stem is the ascending portion of the axis of the plant, developing directly from the plumule,
and bears leaves, branches and flowers. When young, it is normally green in colour. The growing
apex is covered over and protected by a number of tiny leaves which arch over it. The stem often
bears multicellular hairs of different kinds. It branches exogenously and is provided with nodes
and internodes which may not be distinct in all cases. Leaves and branches normally develop from
the nodes and buds. When the stem or the branch ends in a vegetative bud it continues to grow
upwards or sideways. If, however, it ends in a floral bud, the growth ceases.
TYPES OF STEMS
There are varieties of stem structures adapted to perform diverse functions. They may be aerial or
underground. Aerial stems may be erect, rigid and strong, holding themselves in an upright
position, while there are some too weak to support themselves in such a position. They either trail
on the ground or climb neighbouring plants and other objects.
1. Erect or Strong stems e.g.
i.
Caudex is an unbranched, erect, cylindrical and stout stem, marked with scars of
fallen leaves as found in palms.
ii.
Culm is a jointed stem with solid nodes and internodes as found in bamboo.
iii.
Some herbaceous plants, particularly monocotyledons, have no aerial stem.
The underground stem in them produces an erect unbranched aerial shoot bearing either a single
flower or a cluster of flowers; such a flowering shoot is called scape, as in onions.
2. Weak Stems: these are commonly of three kinds
A. Trailers: trailers are those plants whose thin and long or short branches trail on the ground,
with or without rooting at the nodes. When such plants lie prostrate on the ground they are said to
be
i.
Prostrate or procumbent, e.g. Oxalis.
ii.
When the branches of such plants after trailing for some distance tend to rise at their
apex they are said to be decumbent e.g. Tridax.
iii.
When the plants are much branched and the branches spread out on the ground in all
directions, they are said to be diffuse e.g. boerhavia.
B. Creepers: Are weak-stemmed plants with long or short branches creeping along the ground
and rooting at the nodes. A creeping stem may be a runner, stolon or sucker according to its varied
nature.
C. Climbers: climbers are those plants that attach themselves to any neighbouring object, often
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by means of some special devices e.g. pea, yam etc.
Nodes and Internodes: The place on the stem or branch where one or more leaves arise is known
as the node, and spaces between two successive nodes is called the internode.
Bud: A bud is a young undeveloped shoot consisting of a short stem and a number of tender leaves
arching over a growing apex. In the bud, the internodes have not yet developed and thus, the leaves
remain crowded together, forming a compact structure. The lower leaves are older and larger than
those higher. The normal position of a bud is at the apex of the stem or branch and in the axil of a
leaf. The bud in the former case is known as the terminal or apical bud, and in the latter as axillary
bud. Sometimes some extra buds develop by the side of the axillary bud. These are known as
accessory buds.
Fig 12
Functions of the Stem:
 Main purpose is to bear leaves and flowers and spread them out on all sides for proper
functioning.
 Support of the branches which push forward the leaves and flowers
 Conduction of water, mineral salts
 Storage of water and food
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INTERNAL STRUCTURE OF THE STEM
Fig 13
MONOCOT STEMS
The structure of the monocot stem differs from that of dicot and conifers in two main ways.
1. Vascular tissues are usually organized into separated bundles and these are scattered throughout
the stem instead of being in the cylindrical arrangement, as a result, no distinction can be drawn
between the pith and cortex in some monocots such as bamboo, wheat and other grasses. The
internodal regions are hollow, but even in these plants the vascular bundles are irregularly
arranged.
2. All the cells of the procambial strands mature into xylem and phloem. The cambium is therefore
absent since they lack a lateral meristem. The tissues of a monocot stem are primary in origin.
Growth in Vascular Plants
Two kinds of growth occur in vascular plants. These are primary and secondary growth. Primary
growth is initiated in the apical meristem in the shoot and the root.
There are three sections of development
Section A: includes the shoot apex where cell division is dominant but some cell enlargement and
differentiation are also dominant.
Section B: cell division continues but cell enlargement and tissue differentiation are more
conspicuous.
Section C: cell enlargement continues but, more tissues at this point have become differentiated
and tissues such as epidermis, pith and cortex are approaching physiological stages. This primary
growth is completed when derivative of the apical meristem has differentiated into the mature
tissue which constitute the primary body.
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Tissues of the Primary Body
These are the primary phloem and xylem, the cortex, pith and epidermis. These primary tissues
are derived from three tissue systems which can be found below the apical meristem.
1. The dermal tissue is called protoderm which forms the epidermis
2. The meristematic vascular tissue is the procambium which gives rise to the primary vascular
tissues.
3. The ground meristem produces the ground tissues which are the pith and cortex.
VASCULAR TISSUE
The procambium is composed of vertically elongated cells arranged in dicot and coniferous stem
in a cylindrical pattern. The outer cells of the procambium strand differentiate into primary
phloem and the inner into primary xylem. Between the two there are remains of a single layer of
cells called the vascular cambium which does not become differentiated but remains
meristematic. These cells retain indefinitely their capacity to divide to form new cells. Thus causes
increase in diameter of stem.
PRIMARY XYLEM TISSUES
Fig. 14
The first form of primary cells is termed annular elements. The first formed or inner most of the
primary xylem cells are termed annular elements because the secondary wall is laid down as rings
or annular thickens. Others which differentiate later are spiral, scalariform, reticulate and pitted.
All the above types of primary xylem cells don’t necessary occur in a particular plant or plant
organ.
PRIMARY PHLOEM TISSUES
The primary phloem often includes a considerable amount of thick walled supporting elements
called sclerenchyma usually in the form of elongated fibres. Food may be stored in the cortex of
herbaceous stem and collenchyma cells may also occur in the cortex which is able to manufacture
food because they contain chlorophyll. Collenchyma is also an important primary tissue which
supplements the mechanical support function of the vascular tissue.
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SECONDARY GROWTH
Secondary growth results from the activities of a lateral meristem mainly the vascular cambium.
It is characterized by thickening of stem and roots and the tissues formed during such growth are
called secondary tissues. The cambial cells divide and produce secondary xylem (wood) on the
inner side and secondary phloem (bark) on the outside. By the end of the growing season the
first growth or annular ring has been formed.
Fig. 15 Secondary growth of dicotyledonous stem
COMPARISM BETWEEN PRIMARY AND SECONDARY GROWTH
Primary growth is responsible for increase in height and secondary growth for increase in diameter.
Primary tissues make up a relatively small amount of the volume of woody stem and they play an
insignificant role in the economy of the plant. In many herbaceous plants however, primary tissues
constitute the bulk of the plant body.
In monocot a cambium is usually absent and the plant tissues are thus primary in origin. Secondary
xylem tissues include vessels, tracheids, xylem parenchyma cells and xylem fibres. The proportion
of the different cell types and their arrangement vary from species to species.
Hard wood contains many fibres while soft wood contains a great deal of xylem parenchyma and
few fibres.
*Reading Assignments (heartwood, sapwood, growth rings, sieve tubes, phloem fibres, cork
(periderm), lenticels)
HEARTWOOD AND SAPWOOD
A large tree may have a large volume of secondary wood, but only the youngest outermost part of
the secondary wood called the sapwood is alive and functioning. In the sap wood the xylem and
ray parenchyma cells are living and contain stored starch and the vessels are transporting water
from the roots to the leaves. The older central part of the wood is dead. This is the heart wood and
it makes the most valuable timber.
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Fig. 16
GROWTH RINGS
Growth rings appear as concentric circles in cross section and are usually 1 to 10 millimeters wide.
Each growth ring increases the diameter of the trunk; once formed, each ring remains unchanged
in size or position during the life of the tree. Because climate (especially the availability of water)
strongly influences the formation of growth rings, a cross section of wood is a diary of the climatic
history of a particular region. (The science of interpreting history by studying growth rings is called
dendrochronology).
Cambial action in tropical tree is less related to the seasons and more affected by variation in
rainfall throughout the year.
There are several kinds of phloem cells and this make up the inner portion of the bark just outside
the cambium. Secondary phloem is composed of sieved tubes, companion cells, phloem
parenchyma, and phloem fibre.
SIEVE TUBES
In flowering plants these are the most important cells for it is through them that vertical conduction
of food occurs. They are composed of a number of sieve tube elements placed end to end. The
primary walls are relatively thick and the secondary wall is usually absent. The most conspicuous
feature of these cells is the presence of sieve areas which are sunken wall areas with clusters of
pores.
It is through pores sieve plates that food move from one sieve element to another.
The functional life of the sieve tube elements is short, commonly only a single growing season.
The contents become disorganized and in many plants, the sieve tubes eventually collapse as a
result of pressure of surrounding cells, but in long lived monocots with no cambium they depend
upon the original primary sieve tubes in the older portion of the stem.
Associated with each sieve tube may be seen a smaller cell. This is the companion cell. The rest
of the phloem is packed with small-celled parenchyma known as the phloem parenchyma. All the
phloem elements are living, and contain various kinds of food material.
PHLOEM FIBRES
In herbaceous stems, these are usually restricted to the outer part of the phloem. These fibres from
stem of herbaceous dicotyledons are important raw materials from which cord and textiles are
manufactured.
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CORK (PERIDERM)
Most woody and some herbaceous plants form a layer of cork on the outside of the stem. The cork
is formed by cork cambium called phelogen which arises in the epidermis or more commonly in
the outermost layers of the cortex just beneath the epidermis. The cork cambium divides repeatedly
and produces a layer of cork cells to the outside and a few parenchyma cells to the inside. The cork
separates the epidermis from its supply of food and water and so the epidermis dies and gradually
sieves away. Later the cork cells die and become filled with waxy materials. The cork retards the
loss of water from the stem tissues. It is also formed during the healing of wounds thus preventing
the drying out of exposed tissues and the entrance of decaying fungi. The cork may serve as
insulator and may prevent the destruction of the cambium by ground fire.
LENTICEL
These are structures in the bark that serve in aeration. They are composed of masses of loosely
arranged cells with numerous intercellular spaces through which oxygen may penetrate to the
living tissues within. It is the product of the cork cambium which locally gives rise to the cells of
the lenticels instead of cork.
STEM MODIFICATION
The main functions that modified stems perform are:
1. Perenation i.e. surviving from year to year through bad season by certain underground
stems. E.g.
Subterranean stems: In many plants, the aerial parts or branches arise from underground stems;
such stems usually serve as food storage sites and are important sources of food for men and
animals.
a. Rhizome: They are perennial stems horizontal in position. It is the main axis of the shoot with
relatively short and thickened internodes, rich in accumulated food. The tip of each rhizome
ultimately grows up and forms a leafy branch while lateral branches continue to grow underground
e.g. ginger (Zingiber officinale).
b. Stem tubers: The iris potato (Solanum tuberosum) is a good example of stem tubers. Axillary
shoots or stolons are formed underground and grow out horizontally or downwards. The tip of
each shoot swells with stored food, forming a tuber (tuber means a swelling). The surface of the
tuber is marked at one end by a scar which was the point of attachment of the stolon and by the
‘eyes’ scattered over the surface. A single eye consists of a ridge bearing a minute scale-like leaves
in the axil of which are several usually minute buds. The scale leaf and its axillary buds correspond
to the leaf of a woody twig with the bud in the axil of the leaf. The portion of the tuber between
consecutive eyes represents the internode.
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Fig. 17
c. Corms: Is an underground stem that is short, greatly swollen, fleshy and grows in the vertical
direction e.g. Colocasia sp and cocoyam (Xanthosoma). A corm has several nodes each with a
scale leaf. There is an apical bud at the end and this grows out at the start of the growing season to
form a leafy plant, buds in the axils or scale leaves then form new corms which will be ready to
produce new leafy shoots at the beginning of the next growing season.
Fig. 18
d. Bulb: This is an underground modified shoot (rather a single often large, terminal bud)
consisting of a very short convex or slightly conical stem rapped in thickened fleshy scales or leaf
bases which are the site of food accumulation. New bulbs are formed at the base of a shoot and
from axillary buds e.g. Allium cepa (onion)
Fig. 19 Vertical cut section of an onion
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e. Suckers: These are formed from axillary buds which are formed from the under surface of the
soil and grow obliquely upwards and directly gives rise to leafy shoot or a new plant. Example
musa spp. (banana and plantain) which forms suckers which grow up around the parent plant and
takes its place when it has fruited and die.
2. Vegetative propagation by certain horizontal sub-aerial branches spreading out in
different directions. E.g.
Prostrate branches: Many species that have upright shoots produce slender horizontal branches
from basal node of the stem. These structures are called runners or stolons. They grow either over
or under the soil surface. The term runner is usually restricted to adventitious branches and
horizontal branches in the soil. In both runners and stolons nodes and internodes are present and
adventitious roots and sometimes buds are formed at the nodes. They are important in vegetative
reproduction.
Fig. 20
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3. Specialized functions by certain metamorphosed aerial organs. E.g.
Climbing stems: Examples are pole bean, yam and many vines in tropical rain forest. Some of
these species merely leans upon other plants, fences and rocks. Others climb by coiling around a
support or by special modification such as tendrils.
NB:
Branching: In many plants especially dicots, the lateral buds develop into lateral shoots or
branches, but in cereals and other grasses this growth in plant is modified. In these plants, the main
axis of the shoot usually produces branches only from the basal nodes i.e. those close to or just
beneath the surface of the soil. Lateral buds in the axils of the leaves at the base of the main shoot
give rise to secondary aerial branches. The secondary shoots intend may produce branches of a
third order from basal nodes, this process may continue until a considerable number of branches
are formed. This method of branching i.e. from basal nodes is called tillering and the resulting
branches are known as tillers (e.g. rice plant). The number of tillers formed depends on heredity
or variety, closeness of planting or spacing and environmental factors such as moisture availability.
Tillering and branching are of great agronomic importance and the agriculturist depends on this
reaction to multiply number of heads, pods or ears per plant.
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