JNTU Pharm. D I Year REMEDIAL BIOLOGY Previous Question Paper (January/February - 2022 - R08)

Underground stems are modified to perform functions such as perennation (surviving unfavorable conditions), vegetative propagation, and storage of food. These modifications are classified into four main types:

1. Rhizome: A thick, fleshy, horizontal stem that grows underground. It possesses distinct nodes and internodes, scale leaves at nodes, and adventitious roots. Examples include ginger (Zingiber officinale) and turmeric (Curcuma longa).
2. Tuber: The swollen, terminal end of an underground branch. It stores starch and lacks a definite shape. The surface features 'eyes,' which are axillary buds capable of producing new shoots. A classic example is the potato (Solanum tuberosum).
3. Bulb: A highly reduced, disc-like stem where the internodes are extremely shortened. It is surrounded by fleshy scale leaves that store food. Bulbs can be tunicated (e.g., onion, where leaves form concentric layers) or scaly (e.g., garlic, where leaves overlap like scales).
4. Corm: A condensed, vertical, fleshy underground stem that grows upright. It is usually spherical or flattened and contains nodes and internodes with scale leaves. Unlike bulbs, corms are solid and do not have fleshy leaves. Examples include Colocasia and Gladiolus.

These modifications allow plants to remain dormant during harsh environmental conditions and regenerate rapidly when favorable growth conditions return.

Cross-pollination, or allogamy, is the transfer of pollen grains from the anther of one flower to the stigma of another flower on a different plant of the same species. To ensure cross-pollination and prevent self-pollination, plants have evolved several structural and functional mechanisms known as contrivances or devices.

The primary methods are as follows:

• Dicliny (Unisexuality): Flowers are either staminate (male) or pistillate (female). In monoecious plants (e.g., maize, castor), both types occur on the same plant, while in dioecious plants (e.g., papaya, date palm), they occur on separate plants, making cross-pollination mandatory.
• Dichogamy: This involves the maturation of male and female reproductive organs at different times. In protandry (e.g., sunflower), anthers mature before the stigma. In protogyny (e.g., Ficus), the stigma matures before the anthers.
• Prepotency: Pollen from a different flower germinates much faster on the stigma than pollen from the same flower, ensuring that cross-pollinated seeds are produced preferentially.
• Heterostyly: Flowers exhibit different lengths of styles and stamens. In distyly (e.g., Primula), some flowers have long styles and short stamens, while others have short styles and long stamens, physically hindering self-pollination.
• Herkogamy: A mechanical or physical barrier exists between the anther and the stigma of the same flower. For example, in pansies, the stigma is covered by a flap (the rostellum) that prevents self-pollen from reaching it.
• Self-Sterility (Self-Incompatibility): A genetic mechanism where pollen grains from a flower fail to germinate or grow on the stigma of the same flower due to the presence of inhibitory substances or genetic incompatibility (e.g., tobacco, potato).