The following points highlight the three main types of modifications of stems. The types are: 1. Sub Aerial Modifications 2. Underground Modifications 3. Aerial Modifications or Metamorphosed Stems.
Type # 1. Sub Aerial Modifications:
Slender branches develop from the axils of the leaves of some plants which usually run horizontally on the surface of the soil. Most of them are sub aerial and some may be partly subterranean. Their main function is vegetative multiplication. As a matter of fact, sub aerial modified stems are instrumental in very rapid multiplication of certain plants.
(A) Runner or Stolon:
It is a slender branch which develops from the axil of a leaf and grows prostrate on the surface of the soil. It has nodes and long internodes. Roots develop at the nodes to fix the runner to the soil, and the buds at the axils of the scale leaves grow into aerial shoots which, again, produce runner to behave in the same manner.
Thus quite a good number of runners are formed from the mother plant (Fig. 54, A & B). Each of them can lead independent existence if they get severed from the mother plant. Examples—Oxalis (B. Amrul). Hydrocotyle (B. Thulkuri).
It is a short condensed runner. It also develops from the axil of a lower leaf of the mother plant as a short slender branch which produces roots and a rosette of leaves from the tip (Fig. 54, C), e.g. water hyacinth (B. Kachuri pana), Pistia (B. Bara pana).
It is a branch which grows to certain extent beneath the surface of the soil, then comes upwards and produces the green aerial portions (Fig. 55). Adventitious roots arise, as usual, from the subterranean portion. Examples — Chrysanthemum (B. Chandra- mallika), Mint (B. Pudina).
Type # 2. Underground Modifications:
Unlike normal stems, they grow beneath the surface of the soil and are non-green in colour, so they are often confused with roots. Presence of nodes and internodes, scale leaves at the nodes, axillary and terminal buds are distinct morphological characters to show that they are stems and not roots. Underground modified stems are usually fleshy due to storage of food matters.
Their main functions are:
(i) Perennation or tiding over the unfavourable period of the year,
(ii) Storage of food matters, and, lastly,
(i) Vegetative multiplication.
Type # 3. Aerial Modifications or Metamorphosed Stems:
In some plants the stems undergo extreme modifications resulting in the change of structures associated with change of functions. They are called metamorphosed stems.
The following are the types:
1. Thorn (Fig. 59, C & D):
It is a hard sharp-pointed structure used as a defensive organ. It develops from the axil of the leaf, the normal position of a branch. The growth of that branch is arrested and it is converted into a hard pointed body for self- defence.
Besides axillary origin, it has other morphological characters of stems. Thorn often bears leaves from the nodes, and even flowers. They may be simple or branched. Common example— Duranta (B. Mehdi).
2. Stem Tendrils (Fig. 59, A & B):
Weak plants often produce coiled sensitive bodies, called tendrils, for the purpose of climbing. In passion-flower (B. Jhumkolata), Vitis (B. Harjora), etc., similar tendrils are formed from the axils of leaves or from the terminal position.
So morphologically they are nothing but modifications of stem. Sometimes they also bear scale leaves. (Here the stem tendrils are being discussed; other plant parts like leaves, flower axis may also be modified into tendrils.)
3. Phylloclade or Cladode (Figs. 59, E, F & 60):
In plants growing in very dry places like deserts, the leaf surface is extremely reduced and the stems become green, flattened or rounded in shape. Thus they take up the general appearance and functions of leaves. These peculiar metamorphosed stems are called phylloclades.
Examples —Cacti, Opuntia (B. Fanimanasha). Phylloclade of one internode is called cladode, e.g. Asparagus (B. Satamuli). Phylloclades have nodes and inter-nodes, minute scale leaves or spiny modifications of leaves and flowers.