The wide range in the structure of higher plants will never fail to fascinate us. Even though the angiosperms show such a large diversity in external structure or morphology, they are all characterised by presence of roots, stems, leaves, flowers and fruits.For any successful attempt at classification and at understanding any higher plant (or for that matter any living organism) we need to know standard technical terms and standard definitions. We also need to know about the possible variations in different parts, found as adaptations of the plants to their environment, e.g., adaptions to various habitats, for protection, climbing, storage, etc.If you pull out any weed you will see that all of them have roots, stems and leaves. They may be bearing flowers and fruits. The underground part of the flowering plant is the root system while the portion above the
ground forms the shoot system
THE ROOT :
In majority of the dicotyledonous plants, the direct elongation of the radicle leads to the formation of primary root which grows inside the soil.It bears lateral roots of several orders that are referred to as secondary, tertiary, etc. roots. The primary roots and its branches constitute the tap root system, as seen in the mustard plant . In monocotyledonous plants, the primary root is short lived and is replaced by a large number of roots.These roots originate from the base of the stem and constitute the fibrous root system, as seen in the wheat plant . In some plants, like grass,
Monstera and the banyan tree, roots arise
from parts of the plant other than the radicle and are called adventitious roots.The main functions of the root system are absorption of water and minerals from the soil, providing a proper anchorage to the plant parts, storing
reserve food material and synthesis of
plant growth regulators.
Regions of the Root
The root is covered at the apex by a thimble-like
structure called the root cap . It protects the tender apex of the root as it makes its way through the soil. A few millimetres above
the root cap is the region of meristematic
activity. The cells of this region are very small,
thin-walled and with dense protoplasm. They
divide repeatedly. The cells proximal to this
region undergo rapid elongation and
enlargement and are responsible for the growth
of the root in length. This region is called the
region of elongation. The cells of the elongation zone gradually differentiate and mature. Hence, this zone, proximal to region of elongation, is called the region of maturation. From this region some of the epidermal cells form very fine and delicate, thread-like structures called root hairs. These root hairs absorb water and minerals from the soil.
Modifications of Root :
Roots in some plants change their shape and
structure and become modified to perform
functions other than absorption and conduction of water and minerals. They are modified for support, storage of food and respiration . Tap roots of carrot, turnip and adventitious roots of sweet potato, get swollen and store food. Can you give some more such examples? Have you ever wondered what those hanging structures that support a banyan tree are? These are called prop roots. Similarly, the stems of maize and sugarcane have supporting roots coming out of the lower nodes of the stem. These are called stilt roots. In some plants such as Rhizophora growing in swampy areas, many roots come out of the ground and grow vertically upwards. Such roots, called pneumatophores, help to get oxygen for respiration.
THE STEM :
What are the features that distinguish a stem from a root? The stem is the ascending part of the axis bearing branches, leaves, flowers and fruits. It develops from the plumule of the embryo of a germinating seed. The stem
bears nodes and internodes. The region of the stem where leaves are born are called nodes while internodes are the portions between two nodes.The stem bears buds, which may be terminal or axillary. Stem is generally
green when young and later often become woody and dark brown. The main function of the stem is spreading out branches bearing
leaves, flowers and fruits. It conducts water, minerals and photosynthates. Some stems perform the function of storage of food, support, protection and of vegetative propagation.
Modifications of Stem :
The stem may not always be typically like what they are expected to be.They are modified to perform different functions . Underground
stems of potato, ginger, turmeric, zaminkand, Colocasia are modified to store food in them. They also act as organs of perennation to tide over conditions unfavourable for growth. Stem tendrils which develop from axillary buds, are slender and spirally coiled and help plants to climb such as in gourds (cucumber, pumpkins, watermelon) and grapevines. Axillary buds of stems may also get modified into woody, straight and pointed thorns. Thorns are found in many plants such as Citrus, Bougainvillea. They protect plants from browsing animals. Some plants of arid regions modify their stems into flattened (Opuntia), or fleshy cylindrical (Euphorbia) structures. They contain chlorophyll and carry out photosynthesis. Underground stems of some plants such as grass and strawberry, etc., spread to new niches and when older parts die new plants are formed. In plants like mint and jasmine a slender lateral branch arises from the base of the main axis and after growing aerially for some time arch downwards to touch the ground. A lateral branch with short
internodes and each node bearing a rosette of leaves and a tuft of roots is found in aquatic plants like Pistia and Eichhornia. In banana, pineapple and Chrysanthemum, the lateral branches originate from the basal and
underground portion of the main stem, grow horizontally beneath the soil and then come out obliquely upward giving rise to leafy shoots.
The leaf is a lateral, generally flattened structure borne on the stem. It develops at the node and bears a bud in its axil. The axillary bud later develops into a branch. Leaves originate from shoot apical meristems and
are arranged in an acropetal order. They are the most important vegetative organs for photosynthesis. A typical leaf consists of three main parts: leaf base, petiole and lamina. The leaf is attached to the stem by the leaf base and may bear two lateral small leaf like structures called stipules. In monocotyledons, the leaf base expands into a sheath covering the stem partially or wholly. In some leguminous plants the leafbase may
become swollen, which is called the pulvinus. The petiole help hold the blade to light. Long thin flexible petioles allow leaf blades to flutter in wind, thereby cooling the leaf and bringing fresh air to leaf surface.The lamina or the leaf blade is the green expanded part of the leaf with veins and veinlets. There is, usually, a middle prominent vein, which is known
as the midrib. Veins provide rigidity to the leaf blade and act as channels of transport for water, minerals and food materials. The shape, margin, apex, surface and extent of incision of lamina varies in different leaves.
The arrangement of veins and the veinlets in the lamina of leaf is termed as venation. When the veinlets form a network, the venation is termed as reticulate . When the veins run
parallel to each other within a lamina, the venation is termed as parallel . Leaves of
dicotyledonous plants generally possess reticulate venation, while parallel venation is the characteristic of most monocotyledons.
Types of Leaves :
A leaf is said to be simple, when its lamina is entire or when incised, the incisions do not touch themidrib. When the incisions of the lamina reach up to the midrib breaking it into a number of leaflets, the leaf is called compound. A bud is present in the axil of petiole in both simple and compound leaves, but not in the axil of leaflets of the compound leaf.
The compound leaves may be of two types. In a pinnately compound leaf a number of leaflets are present on a common axis, the rachis, which represents the midrib of the leaf
as in neem.
Phyllotaxy is the pattern of arrangement of
leaves on the stem or branch. This is usually
of three types – alternate, opposite and
whorled. In alternate type of phyllotaxy, a single leaf arises at each node in alternate manner, as in china rose, mustard and sun flower plants. In opposite type, a pair of leaves arise at each node and lie opposite to each other as in Calotropis and guava plants. If more than two leaves arise at a node and form a whorl, it is called whorled, as in Alstonia.
Modifications of Leaves :
Leaves are often modified to perform
functions other than photosynthesis. They
are converted into tendrils for climbing as
in peas or into spines for defence as in cacti
The fleshy leaves of onion and garlic store food . In some plants such as Australian acacia, the leaves are small and short-lived. The petioles in
these plants expand, become green and
synthesise food. Leaves of certain
insectivorous plants such as pitcher plant,
venus-fly trap are also modified leaves.
THE INFLORESCENCE :
A flower is a modified shoot wherein the shoot
apical meristem changes to floral meristem.
Internodes do not elongate and the axis gets
condensed. The apex produces different
kinds of floral appendages laterally at
successive nodes instead of leaves. When a
shoot tip transforms into a flower, it is always
solitary. The arrangement of flowers on the floral axis is termed as inflorescence. Depending on whether the apex gets converted into a flower or continues to grow, two major types of inflorescences are defined – racemose and cymose. In racemose type of inflorescences the main axis continues to
grow, the flowers are borne laterally in an acropetal succession
In cymose type of inflorescence the main axis
terminates in a flower, hence is limited in growth.The flowers are borne in a basipetal order.
THE FLOWER :
The flower is the reproductive unit in the
angiosperms. It is meant for sexual reproduction. A typical flower has four different kinds of whorls arranged successively on the swollen end of the stalk or pedicel, called thalamus or receptacle. These are calyx, corolla, androecium and gynoecium. Calyx and corolla are accessory organs, while androecium and gynoecium are reproductive organs. In some flowers like lily, the calyx and corolla are not distinct and are termed as perianth.
When a flower has both androecium and
gynoecium, it is bisexual. A flower having either
only stamens or only carpels is unisexual.
In symmetry, the flower may be
actinomorphic (radial symmetry) or
zygomorphic (bilateral symmetry). When a flower can be divided into two equal radial halves in any radial plane passing through the centre, it is said to be actinomorphic, e.g., mustard, datura, chilli. When it can be divided into two similar halves only
in one particular vertical plane, it is zygomorphic, e.g., pea, gulmohur, bean, Cassia. A flower is asymmetric (irregular) if it cannot be divided into two similar halves by any vertical plane passing through the centre, as in canna. A flower may be trimerous, tetramerous or pentamerous when the floral appendages are in multiple of 3, 4 or 5, respectively. Flowers
with bracts-reduced leaf found at the base of the pedicel- are called bracteate and those without bracts, ebracteate. Based on the position of calyx, corolla and androecium in respect of the ovary on thalamus, the flowers are described as hypogynous, perigynous and epigynous . In the hypogynous flower the
gynoecium occupies the highest position while the other parts are situated below it. The ovary in such flowers is said to be superior, e.g., mustard, china rose and brinjal. If gynoecium is situated in the centre and other parts of the flower are located on the rim of the thalamus almost at the same level, it is called perigynous. The ovary here is said to be half
inferior, e.g., plum, rose, peach. In epigynous flowers, the margin of thalamus grows upward enclosing the ovary completely and getting fused with it, the other parts of flower arise above the ovary. Hence, the ovary is
said to be inferior as in flowers of guava and cucumber, and the ray florets of sunflower.
Parts of a Flower
Each flower normally has four floral whorls, viz., calyx, corolla,androecium and gynoecium
The calyx is the outermost whorl of the flower and the members are called sepals. Generally, sepals are green, leaf like and protect the flower in the bud stage. The calyx may be gamosepalous (sepals united) or polysepalous (sepals free).gamopetalous (petals united) or polypetalous (petals free). The shape
and colour of corolla vary greatly in plants. Corolla may be tubular, bell-
shaped, funnel-shaped or wheel-shaped.
Aestivation: The mode of arrangement of sepals or petals in floral bud
with respect to the other members of the same whorl is known as
aestivation. The main types of aestivation are valvate, twisted, imbricate and vexillary . When sepals or petals in a whorl just touch
one another at the margin, without overlapping, as in Calotropis, it is said to be valvate. If one margin of the appendage overlaps that of the
next one and so on as in china rose, lady’s finger and cotton, it is called twisted. If the margins of sepals or petals overlap one another but not in any particular direction as in Cassia and gulmohur, the aestivation is called imbricate. In pea and bean flowers, there are five petals, the largest (standard) overlaps the two lateral petals (wings) which in turn overlap
the two smallest anterior petals (keel); this type of aestivation is known as vexillary or papilionaceous.
Androecium is composed of stamens. Each stamen which represents the male reproductive organ consists of a stalk or a filament and an anther. Each anther is usually bilobed and each
lobe has two chambers, the pollen-sacs. The pollen grains are produced in pollen-sacs. A sterile stamen is called staminode.
Stamens of flower may be united with other members such as petals or among themselves. When stamens are attached to the
petals, they are epipetalous as in brinjal, or epiphyllous when attached to the perianth as in the flowers of lily. The stamens in a
flower may either remain free (polyandrous) or may be united in varying degrees. The stamens may be united into one bunch or
one bundle (monoadelphous) as in china rose, or two bundles (diadelphous) as in pea, or into more than two bundles (polyadelphous) as in citrus. There may be a variation in the length
of filaments within a flower, as in Salvia and mustard.
Gynoecium is the female reproductive part of the flower and is made up of one or more carpels. A carpel consists of three parts namely
stigma, style and ovary. Ovary is the enlarged basal part, on which lies the elongated tube, the style. The style connects the ovary to the
stigma. The stigma is usually at the tip of the style and is the receptive surface for pollen grains. Each ovary bears one or more
ovules attached to a flattened, cushion-like placenta. When more than one carpel is present, they may be free (as in lotus and rose)
and are called apocarpous. They are termed syncarpous when carpels are fused, as in mustard and tomato. After fertilisation, the
ovules develop into seeds and the ovary matures into a fruit.
Placentation: The arrangement of ovules within the ovary is known
as placentation. The placentation are of different types namely, marginal, axile, parietal, basal, central and free central .
In marginal placentation the placenta forms a ridge along the ventral suture of the ovary and the ovules are borne on this ridge
forming two rows, as in pea. When the placenta is axial and the ovules are attached to it in a multilocular ovary, the placentaion is
said to be axile, as in china rose, tomato and lemon. In parietal placentation, the ovules develop on the inner wall of the ovary or
on peripheral part. Ovary is one-chambered but it becomes two-chambered due to the formation of the false septum, e.g., mustard
and Argemone. When the ovules are borne on central axis and septa are absent, as in Dianthus and Primrose the placentation is called free central. In basal placentation, the placenta develops at the base of ovary and a single ovule is attached to it, as in sunflower, marigold.
THE FRUIT :
The fruit is a characteristic feature of the flowering plants. It is a mature
or ripened ovary, developed after fertilisation. If a fruit is formed without fertilisation of the ovary, it is called a parthenocarpic fruit.
Generally, the fruit consists of a wall or pericarp and seeds. The pericarp may be dry or fleshy. When pericarp is thick and fleshy, it is
differentiated into the outer epicarp, the middle mesocarp and the inner endocarp.In mango and coconut, the fruit is known as a drupe
They develop from monocarpellary superior ovaries and are one seeded. In mango the pericarp is well differentiated into an outer thin epicarp, a middle fleshy edible mesocarp and an inner stony hard endocarp. In coconut which is also a drupe, the mesocarp is fibrous.
The ovules after fertilisation, develop into seeds. A seed is made up of a seed coat and an embryo. The embryo is made up of a radicle, an embryonal axis and one (as in wheat, maize) or two cotyledons (as in gram and pea).
Structure of a Dicotyledonous Seed
The outermost covering of a seed is the seed coat. The seed coat has two layers, the outer testa and the inner tegmen. The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit. Above the hilum is a small pore called the micropyle. Within the seed.coat is the embryo, consisting of an embryonal axis and two cotyledons. The
cotyledons are often fleshy and full of reserve
food materials. At the two ends of the
embryonal axis are present the radicle and
the plumule . In some seeds such as castor the endosperm formed as a result of double fertilisation, is a food storing tissue. In plants such as bean, gram and pea, the endosperm is not present in mature seeds and such seeds are called non-endospermous.
Structure of Monocotyledonous Seed :
Generally, monocotyledonous seeds are endospermic but some as in orchids are non-endospermic. In the seeds of cereals such as maize the seed coat is membranous and generally fused with the fruit wall. The
endosperm is bulky and stores food. The outer covering of endosperm separates the embryo by a proteinous layer called aleurone layer. The embryo is small and situated in a groove at one end of the endosperm. It consists of one large and shield shaped cotyledon known as scutellum and a short axis with a plumule and a radicle. The plumule and radicle are enclosed in sheaths which are called coleoptile and coleorhiza respectively.
DESCRIPTION OF SOME IMPORTANT FAMILIES
This family was earlier called Papilionoideae, a subfamily of family Leguminosae. It is distributed all over the world
Trees, shrubs, herbs; root with root nodules
Stem: erect or climber
Leaves: alternate, pinnately compound or simple; leaf base,pulvinate; stipulate; venation reticulate
Flower: bisexual, zygomorphic
Calyx: sepals five, gamosepalous; valvate/imbricate aestivation
Corolla: petals five, polypetalous, papilionaceous, consisting of a posterior
standard, two lateral wings, two anterior ones forming a keel (enclosing stamens and pistil), vexillary aestivation
Androecium: ten, diadelphous, anther dithecous
Gynoecium: ovary superior, mono carpellary, unilocular with many ovules, style single
Fruit: legume; seed: one to many, non-endospermic
Many plants belonging to the family are sources of pulses (gram, arhar,sem, moong, soyabean; edible oil (soyabean, groundnut); dye (Indigofera);fibres (sunhemp); fodder (Sesbania, Trifolium), ornamentals (lupin, sweet
pea); medicine (muliathi).