This is the part of the plant which grows above the soil except the modified aerial roots. The shoot system consists of two main organs – the stem and the leaves. During the reproductive stage, flowers and fruits may be seen on different branches of the stem
A stem consists of nodes and internodes alternating throughout its length. It is more or less cylindrical in appearance and ends in a terminal bud. Nodes are the slightly enlarged portions of the stem from which leaves and buds arise. The buds occur on the axil of leaves and sometimes develop into branches and flowers. An internode is the portion between two successive nodes. The terminal bud consists of actively dividing cells (meristematic cells). All the young tissues of the shoot system develop from these meristematic cells. Newly formed leaves can be seen covering the terminal buds.
Types of stems
Different types of stems are seen in flowering plants. With regards to their external characteristics and internal structures, stems can be grouped into the following:
1) Woody stems
These are the stems of trees and shrubs. In this group the xylem tissue is highly developed into wood which occupies a very large area of the stem, hence the stem cannot easily be bent.
2) Herbaceous stem
In this group the stems are thin. Wood is absent or present in very small amount, hence the stems can easily be bent. Woody and herbaceous stems belong to the dicot plants.
3) Monocotyledonous stems (monocot stems)
These are the type of stems found in grasses, maize, lily and palms. Their vascular bundles are scattered and there is no secondary growth.
4) Other types of stem
Other types found within the dicot and the monocot plants are:
Leaning stems (Twiners)
These are tender stems with no means of support. They therefore grow by twisting round other plants for support. An example is the aerial stem of yam.
5) Creeping stems
These are stems growing on the soil surface. They spread in all directions, producing roots at their nodes and leaves which cover the ground. A good example is seen in sweet potato.
6) Modified stems
The normal or unmodified stems grow upright above the ground. Some stems are however modified for different purposes and functions. These are:
These are thin leafless and spirally curled branches with which certain plants get attached to neighbouring objects (fig. 2.16).Tendrils are therefore modified for climbing. They occur in passion flower, Curcubita and melon.
b) Thorns and spines.
These are outgrowths of the stem. Thorns are from axillary buds hence they are borne on the axils of leaves. Thorns are seen in orange trees. Spines appear on the stem body. Thorns and spines are stems modified for protection against animals.
c) Underground stems
These are the stem tubers, such as rhizomes, corms and bulbs modified for food storage and vegetative reproduction.
Runners, offsets, suckers and stolons are various types of stems also modified for vegetative reproduction. These organs are fully dealt with in the chapter on storage organs and vegetative reproduction.
Functions of stems
- They hold the leaves in the best position for light which is used in photosynthesis.
- They hold the flowers and fruits in the best position for pollination and dispersal respectively.
- They conduct water and mineral salts from the roots to the leaves and manufactured food from the leaves to the roots, growing regions and storage organs of the plant.
- Some stems are modified for special functions such as climbing, protection, food storage, water storage and vegetative reproduction.
- Young parts of stems contain chlorophyll, hence they take part in photosynthesis.
A leaf is the flattened outgrowth from the nodes of stems and their branches. A complete leaf bears a bud on its axil. A leaf is usually greenish in colour. In many dicot plants, the leaves are attached to the stem or branch by a stalk or petiole which continues in the lamina as mid-rib, branching into veins.A dicot leaf has a net-work or reticulate venation. Many monocot leaves have no petioles. The leaves are attached to the stem by sheathing leaf-base almost encycling the entire stem. Monocot leaves have parallel venation along the sides of the mid-rib.
Yam is however an exception. Yam has reticulate venation like dicot leaves.
In banana (a monocot) the veins branch from the mid-rib almost at right angles to it, but are parallel to each other.
There are a lot of variations in the overall shape of the leaves. These variations appear in the leaf-blade or lamina, the apex, the margin and the base. The margin for example may be smooth or entire, serrated or lobed. The familiar leaf shapes are shown in figure 2.20.
A) Sagittate or arrow-shaped as in arrowhead.
B) Linear as in grass.
C) Oval as in Ixora.
D) Ovate as in Hibiscus.
E) Oblong as in banana.
F) Cordate or heart-shaped as in yam.
Forms of leaves
There are two forms of leaves. These are simple and compound leaves.
1) Simple leaves
A leaf is said to be simple if it consists of only one leaf-blade. The leaf-blade may however become deeply lobed, but this does not extend to the mid-rib or petiole. Sometimes the mid-rib of a deeply lobed simple leaf could be divided at the base where it joins the petiole,to form a simple palmate leaf as in pawpaw and cassava.
(Note that the lobing does not extend to the midribs). Every complete leaf has on its axil, an axillary bud which sometimes develops into a branch shoot (axillary shoot).
2) Compound leaves
A compound leaf is a leaf divided into separate laminated units known as leaflets. A leaflet has its own leafbase and is either attached to a mid-rib (rachis) or directly to the petiole head. Leaflets have no axillary buds.
Types of compound leaves
a) Pinnate leaf: The leaflets are arranged on both sides of the midrib or rachis.
There are two forms of pinnate leaves.
- Imparipinnate: The apex of the leaf terminates in a single leaflet. An imparipinnate leaf is made up of only three leaflets is trifoliate.
- Paripinnate: The apex of the leaf terminates in two leaflets.
b) Bipinnate leaf: This is a compound leaf in which the rachis gives off opposite branches or pinnae which bear oppositely arranged leaflets.
In other words the pinnate is branched twice, as in Pride of Barbados, sensitive plant and Delonix regia.
A major difference between pinnate and bipinnate leaves is that in a pinnate leaf the leaflets are attached to the rachis or midrib, whereas ina bipinnate leaf the leaflets are attached to the pinnae which are branches on the rachis.
c) Digitate (compound palmate) leaf
In this type the leaflets radiate from the head of the petiole as in silk cotton, and umbrella tree (Mussanga cecropoides).
Arrangement of leaves
These are the various forms in which leaves are attached to the stems. The usefulness of leaf arrangement is to prevent one leaf from shading another so that each leaf receives maximum amount of sunlight for photosynthesis. The arrangement of leaves is also very useful in the classification of plants.
When the leaves appear singly on each node on either side of the stem (left and right), the arrangement is alternate as in Crotalaria and sour sop.
In some plants, two leaves appear from each node on the opposite sides of the stem, this is opposite arrangement as in guava.
When the leaves appear singly on each node but from any part of the stem, it gives a spiral arrangement as in pawpaw, cassava, and palms.
Sometimes opposite arrangement may be modified into decussate, in which case the opposite leaves on one node are at a right angle to the opposite set of leaves below them. Examples are seen in Aspilia and independence plant (Chromolaena odorata). Ixora is another good example of decussate arrangement.
In another form of arrangement more than two leaves occur at each node as in Allamanda and Alstonis. This is whorled arrangement.
Functions of leaves
The normal functions of leaves are:
- Respiration (gaseous exchange).
- Transportation of water and food.
Other special functions of leaves include:
- Protection: The leaves of some plants such as cactus and palm fronds of palm trees are modified into spines for the protection of the plants.
- Climbing: Some plants such as Gloriossa superba have the apex of its leaves modified into tendril for climbing.
- Storage organ: The leaves of some plants such as the aloes are fleshy and thick. This helps in storing water which is needed during the period of drought in desert condition. Food is also stored in the leaves of onion bulbs.
- Feeding: The leaves of some such as sundew (Drosera rotundifolia) and venu’s flytrap (Dionaea muscipula) are modified for trapping insects for food.
- Vegetative reproduction: In some plants such as life plant (Bryophylum pinnatum), vegetative reproduction is by the leaves.
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