(i) Multiplication phase – At sexual maturity, the
undifferentiated primordial germ cells divide
several times by mitosis to produce a large
number of spermatogonia or sperm mother cells.
Spermatogonia (2N) are of two types: Type A
spermatogonia and type B spermatogonia.
Type A spermatogonia serve as the stem cells
which divide to form second type of spermatogonia
whenever required.
Type B spermatogonia are progenitor cells which
function as precursors of spermatozoa.
(ii) Growth phase - Each type B spermatogonium
actively grows to a larger primary spermatocyte by
obtaining nourishment from the nursing cells.
(iii) Maturation phase - Each primary spermatocyte
undergoes two successive divisions, called maturation
divisions. The first maturation division is reductional
or meiotic. Hence, the primary spermatocyte divides
into two haploid daughter cells called secondary
spermatocytes. Both secondary spermatocytes
now undergo second maturation division which is
an ordinary mitotic division to form four haploid
spermatids, by each primary spermatocyte.
The transformation of spermatids into spermatozoa
is called as spermiogenesis or spermateliosis. The
spermatozoa are later known as sperms. Thus, four
sperms are formed from one spermatogonium.
- (i) Plant breeding is an applied branch of botany
which deals with the improvement of cultivated
varieties (cultivars) of plants. It deals with the
improvement in the heredity of crops and production
of new crop varieties which are far better than
original types in all respects. E.g., production of
insect resistant, high yielding and /or early maturing
varieties.
(ii) It is possible to induce mutations artificially through use
of chemicals or radiations (like gamma radiations), and
selecting and using the plants that have the desirable
character as a source in breeding. This process is called
mutation breeding.
Mutation breeding has proved advantageous in many
cases such as in mung bean, resistance to yellow mosaic
virus and powdery mildew were induced by mutations.
(iii) Breeding of crops with higher levels of vitamins and
minerals or higher protein and healthier fats is called
biofortification. This is the most practical aspect to
improve the health of the people.
Plant breeding is undertaken for improved nutritional
quality of the plants with the objectives of improving :
- protein content and quality
- oil content and quality
- vitamin content
- micronutrient and mineral content.
Maize hybrids that had twice the amount of the amino
acids–lysine and tryptophan, compared to existing maize
hybrids were developed in 2000. Wheat variety with high
protein content, Atlas 66 has been used as a donor for
improving cultivated wheat.
OR
(i) Farmyard manure is made up of dung of farm animals,
urine, farm refuse and crop residues which are allowed to
partial decay with the help of soil microorganisms. These
microorganisms decompose complex organic debris into
a dark amorphous substance (humus) and degradation
products which are easily assimilated by plants.
Biofertilisers are the microorganisms such as bacteria,
cyanobacteria, fungi etc., which bring about soil
nutrient enrichment, maximise the ecological benefits
and minimise the environmental hazards. During the
preparation of green manure, many leguminous and non-
leguminous crops are grown in the field and ploughed
- Scala media Pg. 47
- Abducens Pg. 59
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