JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD Pharm. D I Year REMEDIAL BIOLOGY Previous Question Paper (June/July - 2019 - R08)

Underground stems are modified to perform functions such as perennation (survival through unfavorable conditions), vegetative propagation, and food storage. These stems remain below the soil surface and are characterized by the presence of nodes, internodes, and scale leaves.

The primary types of underground stem modifications are:

• Rhizome: A thickened, fleshy, prostrate stem that grows horizontally under the soil. It possesses distinct nodes and internodes with scale leaves at the nodes and adventitious roots on the lower surface. Examples include ginger (Zingiber officinale) and turmeric (Curcuma longa).
• Tuber: The swollen, fleshy tip of an underground lateral branch. It does not have a definite shape and lacks basal roots. The surface shows 'eyes,' which are nodes containing axillary buds capable of producing new plants. An example is the potato (Solanum tuberosum).
• Bulb: A highly reduced, disc-like stem where the internodes are extremely short. The stem is surrounded by fleshy, scale-like 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).
• Corm: A condensed, vertical, swollen underground stem that grows upright. It is more or less spherical and stores food, possessing distinct nodes and internodes with scale leaves and axillary buds. Examples include Colocasia and Gladiolus.

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 adaptations known as contrivances:

1. Dicliny (Unisexuality): The production of unisexual flowers. In monoecious plants, staminate and pistillate flowers occur on the same plant (e.g., maize), while in dioecious plants, they occur on separate plants (e.g., papaya, date palm), making cross-pollination mandatory.
2. Dichogamy: The maturation of anthers and stigmas at different times within a bisexual flower. This includes protandry, where anthers mature first (e.g., sunflower), and protogyny, where the stigma matures before the anthers (e.g., Mirabilis jalapa).
3. Prepotency: A condition where pollen grains from another flower germinate more rapidly on the stigma than the pollen grains from the same flower, effectively favoring cross-fertilization.
4. Self-Sterility (Self-Incompatibility): A genetic mechanism where pollen grains of a flower fail to germinate or grow on the stigma of the same flower due to inhibitory substances, preventing self-fertilization (e.g., tobacco, potato).
5. Heterostyly: The presence of flowers with different lengths of styles and stamens. In distyly, there are two types of flowers, and in tristyly, there are three, which physically prevents self-pollination by ensuring pollen from one level does not easily reach the stigma of the same level (e.g., Primula).
6. Herkogamy: The presence of physical or mechanical barriers between the anthers and the stigma of the same flower. For example, in pansies, the stigma is hidden behind a flap, preventing self-pollination.