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Getting to Know Plants
It is a direct prolongation of the plumule. It is negatively geotropic i.e. grows away from the soil. It is normally positively phototropic i.e. grows or bends towards the light. It bears branches, leaves and flowers. It is differentiated into nodes and internodes which may not be distinct in some cases. The space between two successive nodes is called as internode.
The terminal bud is present at the growing in different directions by giving rise to new shoots sideways. If any bud terminal or axillary ends in a floral bud, the growth in the length of the shoot stops. It may however grow sideways by lateral buds.
Forms of Stem
Stems can be of several types depending upon their general form. Broadly speaking they may be strong or weak-stemmed plants. Most of the plants have aerial stems but some have underground stem.
They are tall plants with woody trunk. They are always perennials.Erect herbs and trees are of various types on the basis of the nature of their stems.
1. Excurrent: In this type of the stem grows in a vertical manner and the branches are produced in an acropetal succession as in Pinus.
2. Deliquescent: The tree grows erect but more by means of lateral buds than by the apical bud. As a result of this the tree has a spreading habit, as in neem, banyan etc.
3. Caudex: In palms etc. the stem is of caudex type. It is very long, cylindrical and stout and bears a cluster of leaves at the top.
4. Scape: In some of the monocotyledons aerial stem is absent. The stem remains underground and produces aerial shoots during favourable conditions.
5. Culm: Plants like bamboo have jointed type of stem, i.e., they have solid nodes and hollow internodes.
They are weak-stemmed plants with thin and long stem and are, therefore, incapable of growing vertically without any support. Such plants, therefore, produce certain structures to enable them to climb up the support and are called climbers. They climb the support with the help of tendrils, roots, etc. They are classified on the basis of the nature of their climbing organ. An account of the climbers is given below :
1. Rootlet Climbers: These climbers produce adventitious roots to catch hold the support. The roots may have adhesive discs at their tips or may secrete a sticky juice for the purpose. Sometimes these roots act as a claw or holdfast. Some of the examples are betel, Pothos, Indian ivy, black pepper, tecoma etc.
2 Scramblers: Some climbers produce thorns and spines which act like hooks to help the plant in getting hold of the support. Spines act as hooks in rose and cane. In Bougainvillea and Uncaria the thorns act as hooks whereas in Artabotrys a hook is produced from the flower-stalk. In cat’s nail (Bignonia venusta) the terminal leaflets are modified into curved hooks for the purpose of clinging to the support.
3. Tendril Climbers: Tendrils are thin, leafless and wiry structures. They coil around the support in a spiral manner. Tendril can be modifications of stem, petiole, leaf, leaflet, etc.
Following are some of the common examples of tendril climbers :
a) Stem-climbers – Passion flower (Passiflora), vine, balloon vine (Cardiospermum), Antigonon, Cucurbita etc.
b) Petiole-tendril– Smilax, Clematis, Nepenthes.
c) Leaf tendril – Wild pea (Lathyrus aphaca).
d) Leaflet tendril – Sweet pea (L.odoratus), Garden pea (Pisum sativum), Naravelia.
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