Stem and It’s Structure:
Stem is an aerial part of a plant and develops from the plumule of seeds.
It develops upwards against the force of gravitation and towards the source of light. It possesses definite nodes and internodes.
The growing point of the stem remains at its apex and is represented by terminal bud.
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Modifications of Stem:
The stem may be modified as follows:
Aerial Modifications
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(i) Runner:
It is a specialized slender branch which runs along with the surface of the ground. It bears nodes with long internodes, e.g., strawberry and grasses.
(ii) Stolon:
It is a special type of runner which does not grow horizontally from the beginning but grows upwards like ordinary branches and then arches down to meet the soil, e.g., raspberry and blackberry.
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(iii) Bulbils:
These are specialized buds which become thick and fleshy due to storage of food in their leaves. When matured, they get detached from the parent plant, fall on the ground and each of them grows up into a new plant, e.g., Agave and Cycas.
(iv) Phyllociades (Cladophylls):
They are the stems which take the shape and function of the leaf. They are mostly found in xerophytic plants which suffer from Hygroscopic Water scarcity of water supply, e.g., Opuntia, Ruscus and Asparagus.
(v) Tendrils:
These are highly specialized climbing organs which help the weak stems to grow above the ground by twining around any support it comes in contact with, e.g., Lagenaria (Ghia) and Momordica (Karela).
(vi) Spines:
These are modified branches which become hard and sharply pointed. The plants which grow in dry localities have this modification. This modification enables the plant to reduce its loss of water and protects it against injuries from animals, e.g., Duranta, Citrus and Alhagi.
(vii) Thorn:
Axillary buds are sometimes arrested and transformed into these hard structures. The development of such sharp and hard structures is a characteristic of xerophytes.
(viii) Thalamus:
The thalamus or floral axis is a modified stem.
(ix) Pseudobulb:
In many aerial orchids, one internode of stem becomes fleshy and tuberous. This structure stores up a good quantity of moisture. Such structures are pseudobulb.
(x) Dwarf-Shoot or Spur or Bracliyplast:
In some plants, most leaves are concentrated on special secondary stems located at axillary positions. These secondary stems are very short with very close internodes so that the leaves appear to be in clusters. They are called dwarf-shoot or spur or brachyplast (found in Beriberis).
B. Underground Modifications
(i) Sucker:
It is an underground runner which arises from the main underground stem as a branch. By the death of the older parts of the stem, it gets detached and forms an independent plant, e.g., Mint (pudina) and Chrysanthemum (gule- daudi).
(ii) Rhizome:
It is a stout, elongated, prostrate, thick underground stem. It creeps horizontally beneath the surface of the soil. It is thickened due to the storage of food material in it. Nodes and internodes are well marked, e.g., ginger, turmeric and canna.
(iii) Corm:
It is a large, fleshy underground stem. It is thick and swollen due to the storage of food and bears a number of loose brown scale (leaves) around the apex, e.g., Amorphophallus, Freesia and Crocus.
(iv) Bulb:
It is a reduced underground stem called the disc. The food material is stored either in the basal part of foliage leaves or in the scale-leaves arranged around the disc, e.g., onion.
(v) Tuber:
They are fleshy parts of a plant which store food material. On the surface of tubers are found numerous pits or depressions called eyes, e.g., potato.
Structure of Stem
A. Structure of Stem Apex:
The growing apex of the stem is composed of a mass of actively dividing cells called Meristematic cells.
Below this is found the zone of elongating cells and then a region of maturing cells. A sectional view of apical meristems shows three distinct regions
1. Dermatogens:
It is the outermost single layer of cells which differentiates into the outer epidermis of the stem.
2. Periblems:
It is internal to the dermatogen and is single layered at the extreme apex, but becomes many layered on the sides. It forms the cortical region and the endodermis.
3. Plerome:
It lies internal to the periblem and forms the central part of the apical meristems. It forms the vascular bundles, pith and medullar rays of stem.
B. Internal Structure of Mature Stem:
A mature stem shows the following structures (from outside to the centre):
1. Epidermis:
Epidermis is single layered with cuticularized. Outer walls which form the cuticle. In green stems, the epidermis may have pores known as stomata.
2. Cortex:
It is formed of thin-walled parenchymatous cells and is situated inside the epidermis. It is differentiated into the following parts:
(i) Hypodermis:
It lies immediately below the epidermis and gives strength to the stem. Hypodermis is composed of several layers of collenchymatous cells.
(ii) Cortex:
It is formed of thin-walled parenchymatous cells and is concerned with the storage and assimilation of food.
(iii) Endodermis (Bundle-Sheath):
It is the innermost layer of the cortex having cells which contain starch grains.
3. Pericycle:
It is a zone of sclerenchymatous and parenchymatous cells inside the endodermis. It is many celled in thickness.
4. Vascular Bundles:
The vascular bundles (xylem and phloem) are arranged in a ring inside the pericycle. The adjacent bundles are separated from one another by rows of parenchymatous cells which radiate outward from the centre of the stem and are known as medullar rays. In between xylem and phloem is a strip of Meristematic cells called cambium.
The phloem tissue consists of a large number of sieve tubes with companion cells and a large amount of phloem parenchyma. The xylem consists of trachea, tracheids and xylem parenchyma.
5. Pith:
It forms the central parenchymatous zone of the stem.