give solution of 20 and 21

It includes
1. development of endosperm and embryo,
2. maturation of ovules into seeds
3. Maturation of ovaries into fruits.
 

Formation of Endosperm

• The endosperm develops before the embryo because the cells of the endosperm provide nutrition to the developing embryo.

• The primary endosperm nucleus repeatedly divides to give rise to free nuclei. This stage of development is called free nuclear endosperm.

• Cell wall formation occurs next, resulting in a cellular endosperm.

• The endosperm may be either fully consumed by the growing embryo (as in pea and beans) or retained in the mature seed (as in coconut and castor).

Development of Embryo

• The embryo develops at the micropylar end of the embryo sac where the zygote is situated.

• The zygote gives rise first to the pro-embryo, and then to the globular, heart-shaped, mature embryo.

• A typical dicot embryo consists of an embryonal axis and two cotyledons.

• The portion of the embryonal axis above the level of cotyledons is called epicotyl. It contains the plumule (shoot tip). The portion below the axis is called hypocotyl. It contains the radicle (root tip). The root tip is covered by the root cap. This finally forms the seed.
  
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