MORPHOLOGY OF FLOWERING PLANTS PART-1:MORPHOLOGY OF ROOT, STEM AND LEAF

In this post we are going to discuss the Morphology of Root, Stem and Leaf. For the post of Body Fluids, Click here.

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-

i) absorption of water and minerals from the soil

ii) providing a proper anchorage to the plant parts

iii) storing reserve food material 

iv)synthesis of plant growth regulators.

Regions of Roots:

•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.

Modification of Roots:

•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. 

•The hanging structures that support a banyan tree are called prop roots. 

•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:

•The stem is the ascending part of the axis bearing branches, leaves, flowers and fruits. 

•Stem 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.

Modification 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:

•The leaf is a lateral, generally flattened structure borne on the stem. 

•Leaf 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.

Venation:

•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 the midrib. 

•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. 

•In palmately compound leaves, the leaflets are attached at a common point, i.e., at the tip of petiole, as in silk cotton.

Phyllotaxy:

•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. 

•Leaves 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.