Morphology : Various branches of botany are studied with the help of external morphology.
A typical angiospermic plant is shown in the figure below.
The Root
Root : The root is the descending prolongation of radicle which grows downward into the soil. It does not get light. It seeks moisture. It does not contain chlorophyll so does not undergo photosynthesis and does not develop leaves and buds. It does not have nodes and internodes, but it may produce secondary branches which may again develop branches on them.
Primary or tap root : The primary root is also called tap root is the direct prolongation of the radicle downwards into the soil. It is the main and the first root of the plant. In dicotyledonous plants, the primary root persists, grows and gradually becomes stronger and thicker. It produces branches known as secondary roots or lateral roots. This may again give rise to branches known as tertiary roots. Plants having this type of roots are said to have a tap root system.
Secondary or Tertiary Roots : These are the lateral branches that arise one after the other in a succession from primary roots.
Adventitious root : Adventitious root is one that grows from any part of the plant body other than the radicle.
1. In monocotyledons where the primary root does not exist, a cluster of slender roots is seen to grow from the base of the stem; such roots are called fibrous roots.
2. These roots grow either solitary or inclusters from nodes and even internodes. Adventitious roots are common in bamboo, maize, sugar-cane, betel vine, grass etc.
3. In rose, garden cotton etc, grafting is done in October. Adventitious roots grow on stem cut portions in these plants.
4. These roots may also grow from the petiole or on the vein of a leaf. These are called foliar roots, e.g., Bryophyllum (patharkutchi).
Regions of a tap root : It has following four regions : (i) root cap region, (i) root growing region, (iii) root hair region and (iv) permanent region.
Root cap : Root cap is the cone shaped structure which protects the tender root tip in most plants.
Root hair : The root surface is covered with numerous fine hairs called root hairs. These are celled hairs. Flating plants have roots which are usually without any root hair.
Characteristics of the root
1. The root is the descending part of a plant and the primary root is the direct prolongation of the radicle.
2. Roots grow downwards away from light, that is why they are negatively phototropic in nature.
3. Roots are not normally green in colour and do not contribute anything to photosynthesis.
4. The root does not normally bear buds; sometimes, however, the root may bear vegetative buds for vegetative propagation, e.g., sweet potato.
5. The root cap protects the root tip.
6. The root bears unicellular hairs which occur in a cluster all over the tender part of the young root a little behind the root cap. Root hairs absorb water and mineral salts from the soil to provide nutrition to the plant.
7. Lateral roots always develop from an inner layer, so they are said to be endogenous (endo = inner, gen = producing) in origin.
8. Roots do not have nodes and internodes.
Functions of roots : The root mainly has two basic functions : (i) It provides anchorage to the plant.
(ii) It does the work of absorption, conduction and storage of food materials.
(i) Anchorage : Roots penetrate into the soil and by forming a network in the soil fix the whole plant body firmly in position and the plant does not fall in a storm etc.
(ii) Absorption and conduction : Another basic function of roots is to absorb water and minerals from the soil solution and pass them to the upper part of the plant.
Storage : There are some plants like carrot, radish, beet, sweet potato etc. whose roots store food for future.
The Stem
Stem : The stem is the ascending part of a shoot system which grows towards light and air. It develops from the plumule of a germinating seed.
Characteristics of the Stem
1. The stem is the ascending part of the plant, and is the direct prolongation of the plumule.
2. Stem normally bears leaves, branches. A young stem is green in colour.
3. The stem often bears multicellular hairs of different kinds.
4. The branches develop from the outer surface of the inner tissues.
5. The stem is provided with nodes and internodes. Leaves and branches normally come out from the nodes.
6. When the stem ends in a vegetative bud, it continues to grow in length. If, however, it ends in a floral bud the growth normally ceases.
7. All stems grow towards the source of light, i.e., they are positively phototrophic.
8. The growing apex of stem is covered and protected by a number of tiny leaves. These are on the surface of stem. Stem apex possesses terminal bud.
Normal functions of the stem
(i) Mechanical : It provides support to the branches, leaves, flowers and fruits, and arranges them in such a way that they get maximum light and air.
(ii) Physiological : Stem conducts water and salts to the leaves from the roots. It manufactures food substances from leaves to other parts of the plant. The young stem, which is normally green is capable of manufacturing food. The underground stems of some plants act as storage parts of the plant. For example, potato, ginger, gladiolus, etc. store large quantities of food material in their underground stems.
The Leaf
Leaf : The leaf of a plant is a flat, thin, usually green lateral outgrowth of the shoot system, which arises at a node and bears and bud in its axils.
Leaf-base : The leaf base is the portion of the leaf by means of which it remains attached to the stem or shoot.
Stipules : In many dicotyledonous plants, the leaf base bears a pair of small lateral outgrowths. These are called the stipules.
Petiole : The petiole is the stalk of the leaf which holds the leaf blade in the most favourable position with respect to light.
Leaf blade or lamina : The lamina or leaf blade is the thin, flat, expanded, green portion of a leaf.
Foliage : Green leaves on stem or branches are main photosynthetic organs.
Cotyledons : These are seed leaves present within the seeds they may or may not contain food.
Scale leaves : Scale leaves are colourless leaves, and appear scaly as can be seen in onion.
Bracts : These are special leaves at the base of flowers. From the axil of bracts solitary flower or a cluster of flowers arises. China rose is an example of having bracts.
Floral leaves : Floral leaves are the whorls of flowers like sepals, petals etc.
Dorsiventral leaf : When the leaf is flat, with the blade placed horizontally, showing distinct upper and lower surfaces, it is said to be dorsiventral.
Isobilateral leaf : Isobilateral leaf is the one which is directed vertical upwards. Such a leaf is more or less equally illuminated on both the surfaces by sunlight and therefore, the leaf is unformly green, e.g., grass, wheat, maize and most monocotyledonous plants.
Veins : The veins are thread-like rigid, linear structures which arise from the tip of the petiole and branch repeatedly traversing through the lamina in various directions.
Venation : Venation is the mode of arrangement of veins and veinlets in a lamina.
Reticulate venation : The venation is said to be reticulate when the main vein branches to form a network throughout the lamina again and again. Such type of venation is found in leaves of dicotyledonous plants.
Parallel venation : The venation is parallel when several main veins run approximately parallel to each other and are connected by short cross branches, e.g., grasses. Parallel venation is characteristic of monocotyledonous plants.
Simple leaves : A simple leaf has a single leaf blade or lamina, as in china-rose, mango, cotton, balsam, gourd, etc. it is said to be a simple leaf.
Compound leaf : A compound leaf is one whose leaf blade or lamina is divided into a number of distinct segments.
Functions of leaf
(i) Photosynthesis or manufacture of food is done by leaves.
(ii) Leaves are responsible for transpiration and respiration.
(iii) Leaves store food materials.
(iv) Leaves are responsible for vegetative reproduction.
