Biology Test 350

Gibberellins (GAs) regulate various developmental processes, including stem elongation, germination, dormancy, flowering,flower development and leaf and fruit senescence. When gibberellin is sprayed on sugarcane crop, the length of the stem increases.

They help to initiate rooting in stem cuttings, an application widely used for plant propagation. Auxins promote flowering e.g. in pineapples. They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits

In low concentrations, auxins such as 2,4− D(2,4− Dichlorophenoxy acetic acid) is useful in preventing, pre-harvest fruit drop and leaf drop.

Cytokinin and auxin are two plant hormones that are supplied to the tissue culture medium in definite proportions. They bring about cell division and differentiation of callus. A low auxin to cytokinin ratio promotes shoot formation whereas a high auxin to cytokinin ratio promotes rooting of callus.

Alveoli are tiny air sacs in the lungs where the exchange of gases takes place. They are surrounded by a network of capillaries and do not have cartilaginous rings for support. The bronchial tree, on the other hand, is supported by cartilaginous rings which help to maintain their patency and prevent their collapse. The primary bronchioles, secondary bronchioles, and tertiary bronchioles are part of the bronchial tree and are supported by cartilaginous rings. The trachea, also known as the windpipe, is also part of the bronchial tree and is supported by C-shaped cartilaginous rings

 The diffusion membrane is made up of three major layers (Figure 17.4) namely, the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries and the basement substance (composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer endothelial cells of capillaries) in between them.

The dilation of blood vessels leads to a decrease in peripheral resistance and a decrease in blood pressure, while constriction of blood vessels leads to an increase in peripheral resistance and an increase in blood pressure

The natural system of classification for higher plants was given by Bentham and Hooker in their Genera Plantarum (1862 - 1883). The characters employed in this system include those of study of form (morphology), internal structure (anatomy), development (embryology), reproduction, cell structure (cytology), life processes, (physiology), behaviour and biochemistry.

Auxins inhibit the growth of axillary buds and promote apical dominance. It induces parthenocarpic development of fruits and such fruits are seedless and auxin is also responsible for phototropism and geotropisms. Ripening and maturity of fruits are related to ethylene.

Species is the lowest category in taxonomic hierarchy. Species is a group of animals, plants or other living things that all share common characteristics and that are all classified as alike in some manner.