Q. 25

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Answer :

a. Viable seeds are developed from ovules under suitable conditions. Viability can be affected by many conditions like temperature, humidity, oxygen availability etc. The probable causes that prevented the rest of the 40 ovules from maturing into viable seeds are:


1) Damage to embryo


2) Denaturation of enzymes


3) Exhaustion of food for embryo


4) Excessive dry or wet conditions


5) Extremely high temperature


b. The male sperm nuclei and egg fuse together to form the zygote. This process is called syngamy. The zygote divides into a larger cell called suspensor cell towards micropylar end and a smaller cell called embryonal cell towards antipodal cells. Suspensor is a 6-10 celled structure formed by mitotic division of the suspensor cell. The last suspensor cell is called hypophysis and forms radicle tip. The mitotic division of the zygote gives rise to the pro-embryo first, then to the globular and heart-shaped mature embryo.


A typical dicot embryo consists of two cotyledons and an embryonal axis. The portion of the embryonal axis above the level of cotyledons is called epicotyl and plumule (shoot tip) while the portion below the axis is called hypocotyl and contains the radicle (root tip). The root tip is covered by the root cap.



c. A nutritive tissue called endosperm, surrounding the embryo, is produced during development of seed in flowering plants. During the development of embryo, in some plants the endosperm gets fully consumed. In these cases, the endosperm is absent in the seeds. This is called as an ex-albuminous seed. Examples include pea, grams, bean etc. Whereas the seeds which retain some part of the endosperm, as it does not get fully consumed during embryo development, are called albuminous seeds. Examples include maize, wheat, castor etc.


OR


a. Hormones and their functions



b. Stages:


i. Corpus luteum and placenta co-exist.


Corpus luteum and placenta co-exist during pregnancy where they secrete progesterone and other hormones to maintain the fetus.


ii. Corpus luteum temporarily ceases to exist.


Corpus luteum ceases to exist during menstruation when it degenerates into corpus albicans.


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