What is stem modifications?
To perform specialized functions the stem and the aerial branches get modified in different ways which is called as stem modifications. These modifications are of 3 kinds, viz., underground, sub-aerial and aerial. The sub-aerial modifications are also called metamorphosed stem because their structures are massively modified and sometimes cannot be predictable as stems.
Underground stem modifications
The underground stem modifications form are specialized stems, which are not positively phototropic, they belong to herbaceous plants, acquire various sizes and shapes and store food as carbohydrate in the form of starch. They keep the plant alive during unfavorable condition and with the initiation of favorable condition; they produce the aerial shoot and spread by vegetative means. The general functions of underground stem modifications are:
[i] Storage of foodstuffs.
[ii] They act as per-enacting organs, surviving through unfavorable conditions.
[iii] They act as vegetative propagating organs.
The underground stem modifications are of the following types:
Stem-tuber
Characteristics of this type are as follows:
Potato Plant
The tips of underground creeping branches become swollen and enlarged due to the accumulation of food materials. They are more or less round or oval in shape with compressed nodes and short internodes. The nodes are represented by eye-spots, which are depressions containing scale leaves and rudimentary buds remaining at the axils of scale-leaves and these eye-spots are spirally arranged on the tuber mostly remaining at the distal end. The apical bud is also present at the distal end of this form of stem modifications. The adventitious roots are absent on the ventral surface and it is colorless, and the outer epidermis dark brown to purple in color. Example: potato.
Bulb
Characteristics of this type are as follows:
Stem Modifications of this type get the bulb-like structure with a central basal disc supporting the apical bud. The apical bud is protected by compressed layers of fleshy leaves and outer scaly leaves whereas; the fleshy leaves store carbohydrate and may contain the axillary buds. The apical bud produces the flowering scape during the production of flowers. There is a crown of adventitious roots arising from the ventral surface of the disc and the fleshy leaves are arranged in closed concentric rings, proto-catechuic acid. Example: onion.
There are two types of bulbs, those are as follows:
Tunicate bulbs
Onion
It is the type of stem modifications where the scale leaves are with fleshy leaf bases, which are arranged in concentric manner surrounding the short, disc-like compressed stem. The outer leaves are dry, membranous and form a tunic-like structure. Example: tuberose, onion.
Scaly bulb (Naked bulb)
In this case the thick and fleshy leaf bases of scale leaves are loosely arranged and partly cover each other only at the margins and they are devoid of tunic. Example: garlic, lily.
Rhizome
Stem of Water-lily
Characteristics of this type of stem modifications are as follows:
These are the thick, fleshy, underground stem, which grows horizontally. The nodes and internodes are present, but the internodes are compressed and are covered by brown, papery, scaly leaves. The adventitious, axillary buds are present at the axils of scaly leaves, which account for uneven branching of the rhizome. The apices of the branches contain terminal buds, helping in apical growth and the terminal or axillary buds produce leafy shoots during the growing season which dry of later on, leaving behind a ring-like scar mark on the nodes. There -are slender, thread-like adventitious roots arising from the ventral surface of the rhizome' at the nodes. Example for horizontal rhizome: ginger, turmeric.
Sometime the rhizomes are short, stout, vertical and instead of growing horizontally, they grow vertically downwards. They are called root stocks. Example: arum, water-lily, banana.
Sometimes the horizontal rhizomes are thin and slender, with long internodes and are called sobole. Example of this stem modifications is the grass.
Corm
Characteristics of this type of stem modifications are as follows:
It is a stout, solid, fleshy, club-shaped underground modified stem and they grow vertically or peripherally in the superficial soil. The nodes and internodes are present, nodes are loosely placed and the internodes are reduced and fused to form a single structure. There is a large apical bud and many adventitious buds arising from the axils of scale leaves. The adventitious roots arise from the entire surface of the corm. The axillary buds produce new young corms, which bulges out from all over the corm and accounts for its irregular shape. Example: Amorphophallus.