Question

When a seed of an orange is squeezed, many embryos, instead of one are observed. Explain how it is possible?

Answer 1

Hint: Orange seeds are small and hard to break. While the formation of its seed the nucellus cells surrounding the embryo sac start dividing and protrude into the embryos and form multiple embryos.

Complete answer:
1. Polyembryony can be explained as the occurrence of more than one embryo in seed.
2. In orange, the nucellar cells, integument cells develop into a number of embryos of different sizes.
3. The formation of more than one egg in an embryo sac can cause polyembryony. In short, some of nucellar surrounding the embryo start dividing thus resulting in formation of many embryos.
4. On broad basis polyembryony is of two types:
Spontaneous: It includes naturally occurring polyembryony.
Induced: this type of polyembryony is induced experimentally.

5. Spontaneous embryo is subdivided into two categories:
 (a) True polyembryony: Two or more embryos arise within the same embryo sac from synergid, antipodal or from nucellus integument.
 (b) False polyembryony: Development of embryos in more than one embryo sac in the same ovule.
6. In citrus ( oranges), adventive embryos have been successfully employed to grow these plants. Seedlings formed from nucellar embryos are more vigorous than the clones obtained by repeated vegetative propagation. The nucellar seedlings are superior with respect to:
(i) Well developed tap root system.
(ii) Restoration of vigour.
(iii) Virus free clones.

Note: Causes of Polyembryony: Several theories have been put forward to explain the causes of polyembryony but none of these is satisfactory. According to these theories, polyembryony may be attributed to hybridisation, necrohormone, effect of recessive genes, etc. Neurohormone theory was proposed by Haberlandt (1921, 1922) in Oenothera. It suggests that hormones are released from degenerated cells of nucellus which stimulates the development of adventive embryos.