8. Describe modifications of stem with suitable examples.
The major types of stem modifications are as follows:
1. Rhizome- It is fleshy, non-green underground stem having distinct nodes and internodes. The nodes bear dry scale leaves with axillary buds. Adventitious roots arise from the lower side. e..g., Alocasia, Dryopteris, Banana, Ginger, turmeric, Canna etc.
2. Bulb- It is an underground pyriform to spherical structure bearing a reduced convex or slightly conical disc-shaped stem and several fleshy scales enclosing a terminal bud. e.g., Lily, onion etc.
3. Corm- It is a condensed form of rhizome growing in the vertical direction. It is more or less spherical with a flat base. Adventitious roots arise either from its base or all over the body. Examples- Colocasia, Amorphophallus.
4. Tuber-Stem tuber is a swollen tip of an underground stem. It possesses a number of small depressions called eyes. These eyes represent nodes. Adventitious roots are usually absent e.g., Potato.
5. Runner- These are special, narrow, green, above ground horizontal or prostrate branches which develop at the bases of erect shoots called crowns. E.g. Centella, Oxalis, doob grass etc.
6. Stolon- These are elongated horizontal runners which can cross over small obstacles. The tip of the stolon generally grows above the level of the ground. E.g. Jasmine, peppermint, wild strawberry etc.
7. Offset- They are one internode long small runners which are found in rosette plants at the ground level. E.g Pistia, Eichhornia etc.
8. Stem- tendrils- These are thread like sensitive structures that coil around a support and help the plant in climbing. E.g. grapevine.
9. Stem thorn- These are hard, stiff and sharp structures which protect the plants. E.g. Citrus, Bougainvillea, Duranta etc.
10. Phylloclade- They are flattened or cylindrical green stems of unlimited growth which have taken over the function of photosynthesis. Formation of phylloclades helps the plants to grow in dry habitats. e.g. Opuntia, Casuarina etc.
11. Cladode- These are the green stem of limited growth which have taken over the function of photosynthesis from the leaves. The true leaves are reduced to scales or spines. E.g. Ruscus.