Biology Test 18

Biotic potential is the number of offspring of an individual organism that would survive till reproductive age under ideal conditions.

Genetic diversity is the diversity in the number and types of genes as well as chromosomes present in different species and the variations in the genes and their alleles in the same species. Extensive use of Insecticide and pesticide is the greatest threat to genetic diversity in agricultural corps.

Rhododendrons,are epiphytic species, which grow on other trees or plants, of rhododendrons with limited population are most vulnerable due to loss of the host trees and bird population (limited pollination)

Depending upon the abundance and availability, the natural resources are classified into two categories-inexhaustible and exhaustible. Examples of inexhaustible resources are air, clay and fungi. Excessive use of renewable sources makes them non-renewable, eg, underground water, forests and wild-life are renewable resources but can become non-renewable, if they are not used properly and carefully.

Bulliform cells are large, thin-walled, bubble-shaped epidermal cells that occur in groups on the surface of the leaves of many grasses. When these cells have absorbed water and are turgid, the leaf surface is exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimize water loss.

Collenchyma cells are elongated cells with irregularly thick cell walls that provide support and structure. Their thick walls are composed of the compounds cellulose and pectin. These cells are often found under the epidermis, or the outer layer of cells in young stems and in leaf veins. It produces hypodermis.

A lenticel is a porous tissue consisting of cells with large intercellular spaces in the periderm of the secondarily thickened organs and the bark of woody stems and roots of dicotyledonous flowering plants. It functions as a pore, providing a pathway for the direct exchange of gases between the internal tissues and atmosphere through the bark.

In the dicot root, pith is absent or very small.

In Cucurbita stem, the collenchymatous hypodermis is followed by parenchymatous cortex. . The number of parenchyma layers in the later is more below the ridged and less below the furrows.

If there is a leaf at a node bearing a single branch in its axil, a transverse section of the node shows the vascular system of the main stem, then the branch gap, branch trace, leaf gap and finally the leaf trace.