The part of the sapling which comes out of the seed coat and moves away from the soil or earth is known as Plumule. This plumule further develops into stem which in turn develops into different parts namely the branches, leaves, flowers, fruits, buds, etc. The complete structure excluding the flower and the fruit is known as the vegetative part of the plant. Basically, the stem functions as the pathway for the transport of water, minerals and food for the leaves, flowers and fruits. It is basically characterised by the nodes and internodes and more important are the buds namely the terminal and axial buds. These buds are one of the main characters that differentiate the branch or stem from the rachis present in a compound leaf.
Like the roots even stems undergo modifications to different forms for the purpose of performing special functions and these often become the point of gross identification of plants. Further the floral parts and the microscopical study will give a clear opinion regarding identification. This is due to the fact that the vegetative parts of the plant including the roots undergo drastic changes with respect to the characters like size, colour, thickness, etc according to the environmental conditions that exist. But floral parts being stable their characters are generally considered to be best tool to ascertain the identification including the species. Still, the knowledge about the character of roots are give a considerable input regarding identification of plants.
Generally the stem consist of the main part which gets branched usually in the region of nodes. In the region of nodes the leaves and flowers arise too. The region where the petiole of the leaf attaches to the branch in the region of the node is known as axila where a bud is seen known as the axillary buds. In the tip of a branch or the stem itself there is a bud which becomes the growing end of the stem or branch. This bud is known as the Terminal bud. These buds along with nodes and internodes form the basic characters to differentiate the stem from other parts of the plant. Irrespective of whichever shape or modifications the stem undergoes, the above said characters are seen in them. Like the roots even stems undergo modifications as per the need and environmental conditions. These modifications can be divided into Aerial, Sub-aerial and Underground categories.
MODIFICATION OF STEM
The different modification of stem to serve different purposes under the Aerial category are
- Stem Tendrils – Generally, the axillary buds form new branches, leaves or flowers. But in some cases they get modified into trendrils so as to help them climbing. This is usually seen in weak plants such that they can go upwards with some support. For eg. Grapes
- Phylloclades – Here the stem is flat and fleshy or Cylindrical green leaf-like structure to carry out photosynthesis while the leaves are modified into spines for the purpose of protection. It also facilitates reduction in transpiration. For eg. Cactus
- Hooks and Prickles – They are developed from the nodes and the epidermis which are found on the nodes or internodes. This is having an external origin and is easily detachable. These will not have a connectivity of vascular bundles. Eg. Rose
- Thorns – This modification is developed from the buds (axillary or terminal) and are attached to the stem with a connectivity of vascular bundles. For eg. Lemon
The different modification of stem that come under the Sub-aerial category are
- Runner – This is seen in creepers where the stem is spread over the ground, the roots are developed in each node along with the leaf for providing grip and stability along with optimum use of energy. For eg. Durva grass
- Stolons – In this type of modification, the branches develop from the lower part of the stem and after growing for some distance they make an arch to touch the ground where they give rise to new shoot or a plant. For eg. Jasmine
- Sucker – In this type of stem modification, the mainstem grows in the soil while the branches that grow from nodes come out of the soil. For eg. Peppermint
- Offset – This modification is usually seen in fragile stemmed plants mainly aquatic plants. Here the small plant is produced from the node of the stem like a cloned plant. For eg. Pistia.
The different modification of stem that come under the underground category are
- Tubers – The modification for the purpose of food storage. Here the stem and the branches present in the soil get swollen with the axillary buds are present in the form of eyes and are covered with scaly leaves. For eg. Potato
- Rhizomes – This modification is primarily for the storage of food where the stem is present horizontally below the soil which is swollen. The nodes and internodes are found to be covered with scaly leaves that arise from the nodes. For eg. Ginger
- Corm – The modification of the stem for storing food. The stem is modified into a condensed structure that grow vertically below the soil surface. For eg. Amorphophallus, Colocasia
- Bulb – The stem in this case forms a disc like structure where a numerous fleshy leaves arise from above the disc. The roots arise from the base of the disc. For eg. Onion