Let us know some basic things about stem before going through different stem modifications. Stem meaning- The stem is the ascending part of the axis bearing branches, leaves, flowers and fruits. It develops from the plumule of the embryo from a germinating seed. The stem bears nodes and internodes, and it also bears buds, which may be terminal or auxiliary. Stems are generally green when young and later often become woody and dark brown.
Stem definition - In general, the stem is defined as a long, thin supportive section of the plant or shrubs. It is typically rising above ground but occasionally subterranean.
The main function of the stem is spreading out branches bearing leaves, flowers and fruits. It conducts water, mineral and photosynthesis. Also, some of the stems perform the function of storage of food, support, protection and vegetative propagation.
Stems are not always in the way, we assume them to be, sometimes they are modified to perform different functions. Underground stems of potato, ginger, turmeric, zaminkand, and colocasia are modified to store food in them. They also act as organs of penetration to tide over conditions unfavourable for growth, Stem tendrils which develop from axillary buds, are slender and spirally coiled and help plants to climb such as in grounds (cucumber, pumpkins, watermelon) and grapevines.
Types of Stem Modifications
The three different types of stem modifications are mentioned below -
1. Underground Stem Modifications
From the name, it is very clear that the modification is below the ground level. The main purpose of underground stem modification is for the storage of food, propagation, and penetration. These types of modifications are identified by the presence of nodes and internodes. They help in propagation through asexual reproduction. They also help the plant to survive the period of dormancy. It also provides protection from grazing animals and environmental stress. Different types of underground stem modifications are mentioned below
Rhizome - This type of root runs horizontally to the ground, the upper portion of rhizome produces the scaly leaves and the bottom portion gives rise to adventitious root. Rhizomes contain nodes, internodes, and buds. The main function of the rhizome is to store food. Ginger and turmeric are examples of stem modification in rhizomes.
Bulb - In this type, the stem is reduced and surrounded by scaly leaves. Modified stem examples of bulbs are tulips, lilies, daffodils, onions, and garlic.
Corm - They are the short upright stem, covered with scaly leaves. Corms are harder compared to bulbs. Some of the examples are colocasia, Yam, Saffron.
Tuber - In this, the terminal of the stem is fleshy and stores food and it contains nodes and internodes but due to swelling, they are not very distinct. Presence of scaly leaves at nodes can be seen as eyes or scars in plants like potato, Artichokes.
2. Subaerial Stem Modifications
These types of stems can be seen on the ground or partially on the ground and it can be found in the plants with short lifespan. Those plants have weak and herbaceous roots. In this modification, the stem is used for faster propagation. Some of the aerial modifications of the stem examples are given below
Runner - It starts from the elongation of basal internode, and they run parallel over the ground. Some of the examples of the runners are Grasses, Cynodon, Oxalis.
Stolon - They are the short and weak aerial branch which arises from the main axis. Stolon bends down and touches the ground forming roots. Some of the examples of the stolon are mint, Strawberry.
Sucker - It arises from the underground portion of the stem and it grows horizontally under the soil and then comes above the ground. Suckers develop adventitious roots and shoot with leaves to form a new type of plants. Some of the examples are Chrysanthemum, banana, pineapple.
Offsets - They are found in most aquatic plants. It gives rise to a lateral branch having short internodes. At nodes, rosettes of leaves develop at the upper portion and roots below, some of the examples are Pistia, Eichornia.
3. Aerial Stem Modification
In this type of modification, aerial parts of the stem and buds get modified to perform various functions like support, climbing, photosynthesis, and vegetative propagation. Let us see some of the aerial modification of stem below
Tendrils - They are coiled and a delicate part which is used for climbing. It is developed from the axillary bud, some of the tendrils stem modification examples are grapevines, watermelons, pumpkin, cucumber.
Thorns - They are developed from the axillary buds which are pointed, woody and hard, spiny structures. Ir also provides protection from grazing animals. Some of the examples are Bougainvillea, lemon.
Bulbils - They are the modified axillary bud, which helps to store food and it becomes fleshy. Bulbils detach from the mother plant to develop into a new plant. Example of bulbil is Dioscorea.
Cladode and Phylloclade - They are the modified stems and perform photosynthesis, as it is green. They become fleshy and the leaves are reduced to form spines. They are commonly found in the xerophytic plants to reduce the loss of water. Some of the examples are opuntia, Euphorbia, Asparagus.
In this article, we have come across stem meaning and the different types of stem modifications in brief. We have gone through underground stem modifications, Subaerial stem modifications and aerial stem modification with some examples for stem modifications. After going through all these types of stem modification, It is clear that stems are modified for conducting water, minerals and food to other parts. In some plants, it is modified to perform different functions also mentioned above accordingly.