When stems change their form to perform functions other than the normal functions, it is said to be stem modifications.
Certain stems or branches of the plant change their shapes or forms to carry on special functions such as perennation (to survive during unfavorable conditions, vegetative propagation, storage of food and specialized functions. There are different types of stem modifications based on their position with respect to soil.
(A) Underground modifications:
For the purpose of storage, perennation and vegetative propagation the stems remain under the ground. During unfavorable season, they remain dormant and when the conditions become favorable they give off new aerial shots.
Although non-Greek and are present under the ground, they possess characteristics of stem. These are: presence of nodes and internodes, scale leaves and adventitious roots at the nodes, presence of buds at the axils of scale laves. The following are the different underground modifications of stems based on their shape or forms:
It is provided with nodes, internodes, scale leaves and buds. The stem part creeps horizontally under the ground and is prostrate and thick. It possesses brown scale leaves with itself being brownish often mistaken for root. Adventitious roots are developed on the lower surface of the node.
In turmeric (Curcuma domestica- and ginger (Zingiber officinal- |it is fleshy and branched. The apical bud gives rise to new plants on favorable condition. But the rhizome of Carina is not fleshy. Here the internodes are long and rhizome grows horizontally by the apical bud.
A special type of rhizome is seen where instead of growing horizontally, it grows1 vertically down. It is called root stock, e.g., Alocasia indica.
The axil of lower leaf gives rise to underground branches. Its apices swell up into tubers which are without any definite shape, e.g., potato (Solanum tuberosum). It is a modified stem due to presence of a number of 'eyes' or buds which can give rise to new plants.
The tuber is protected by a corky skin. Adventitious roots are absent. Sometimes, it is very much swollen due to the heavy deposit of food material. Other examples of stem tuber are Cyperus rotundus and Helianthus tuberous.
Bulb is the underground stem which is conical in shape and gives rise to adventitious roots towards the lower side. It produces fleshy scale leaves surrounding an apical bud. It produces the leafless stem, the scape, in favorable season. In this case, food is stored in the fleshy scale leaves.
There are two types of bulbs based on the arrangement of fleshy scale leaves: tunicated bulb and imbricate bulb.
The former possesses a number of concentric scale leaves as found in onion (Allium cepa) and in the latter the scale leaves are not concentric but are arranged loosely overlapping one another only at the margins, e.g., garlic (Allium sativum). In garlic, the fleshy scale leaves remain separate.
It is more or less rounded in shape, a condensed vertical stem with a large apical bud. It Is often somewhat flattened from top to bottom, e.g., Amorphophallus campanulatus. It has nodes, internodes and scale leaves with numerous buds and adventitious roots. It stores up the food material.
(B) Sub-aerial modifications:
The sub-aerial stems are weak and unable to stand straight. They may be partial subterranean. The stem is usually modified for the purpose of vegetative propagation. The different types of sub-aerial modified stems are described as follows.
It is a modified stem produced from auxiliary bud of the leaf. It is slender, prostrate branch with long internodes that creep on the ground. Adventitious roots develop at the nodes. The interode portion is said to be the runner.
Due to the formation of many such runners on all sides the plant spreads out on the ground in all directions. Runners are meant for vegetative propagation, e.g., Oxalis, Marsilea, Dog grass (Cynodon dactylon) and Centella asiatica, etc.
A special kind of underground runner is known as so bole. It is thin and does not store food material, for which it is not included under rhizome, e.g., creeping grassy weed (Agropyron) which grows under the ground in all directions giving rise to numerous runners.
The so bole is divided into small pieces during cultivation in which each node gives rise to a new plant. At the point of contact, the nodes develop adventitious roots and buds. Each such point can give rise to a new plant when detached from the mother plant.
It is a branch developed at the base of the stem under the soil which at first grows upwards and then arches down to meet the soil. A common example is Mentha sp.
This type of modification is found in aquatic plants like Pistia and Eichliornia etc. Just like a runner it develops from the leaf axil that grows horizontally up to some extent. It bears a tuft of leaves towards the upper side and adventitious roots towards the lower side.
An offset is short, soft, prostrate branch with leaves above and a cluster of roots below. It often breaks away from the mother plant and grows independently again producing offset at the leaf-axil.
Like the stolen it is produced from the underground part of the stem. The lateral branch grows obliquely upwards bearing adventitious roots.
It produces leafy shoots when it reaches the soil surface. A sucker is much shorter than a stolon. It strikes roots at the base either before it separates from the mother plant or soon after. A common example is Chrysanthemum.
C. Aerial modifications:
The above ground part of the stem is known as aerial part. Such aerial stems are modified to carry out certain functions. At times, it is metamorphosed due to extreme modifications. There are various kinds of aerial stem modifications.
Tendril is a thin, wiry, leafless, spirally-coiled structure. It occupies the position of a leaf axil or that of a floral bud. It helps in climbing. It is seen in Vitis, passion "flower (Passiflara). In passion flower, it is the axillaries branch that is modified into tendrils.
In balloon vine (Cardiospermum) floral buds are modified to tendril. In Antigonon, it is the floral buds that give rise to tendrils. In Gouania, the branches end in strong tendrils to provide support to the plant
Thorn is a hard, thick, often straight, deep seated and pointed structure. It is a metamorphosed stem because it occupies the position of bud. Mostly it arises at the leaf axil. InDuranta, his axillaries bud which is modified into thorn.
Sometimes thorn also bears small leaves. It is meant to provide protection to the plant. In Carissa, the terminal bud is modified into a pair of thorns. These are distinguished! From spines and prickles being deep seated. In Flacourtia the thorn becomes) branched.
It is a green, flattened or rounded stem provided with nodes, buds and spines. In Opuntia, the leaves are modified into spines. Being green in structure it performs the functions of leaves. In Casuarina it is green and cylindrical and the leaves are reduced to scale leaves. It is the characteristics of xerophytes plants to check the water loss.
The leaves which are mainly responsible for transpiration are either absent or modified. Stem becomes swollen or fattened and green so that it can carry out] photosynthetic function of the leaves. They may contain much mucilage for retaining water. Other examples of phylloclades are Cocoloba and Epiphyllum.
Cladode is a type of phylloclade found in Asparagus. It consists of one inter node only. The ultimate branch lets are of one inter node only. These are cylindrical (not fleshy) and are often mistaken for leaves. They are cladodes while leaves are modified or reduced into prickles. These are characteristic adaptations for desert plants.
It develops at the leaf axil due to modification of vegetative bud or floral bud. It is a roundish multi cellular structure meant for vegetative propagation. It sheds from the mother plant and grows up into a new independent plant, e.g., Dioscorea. In Globba agave and onions, bulbils are seen to occur on the inflorescence being modifications of some flowers.