This is the phenomenon of formation of embryos through asexual reproductive process without the formation of gametes (by gametogenesis) and the act of fertilization.
Agamospermy is of the following three types:
(i) Recurrent Agamospermy (Recurrent Apomixis; Apospory & Diplospory):
In this type of agamospermy, a diploid embryosac is formed either from the diploid nucellar cells (a phenomenon called apospory, e.g., citrus, mango, prickly pear etc.) or from the diploid megaspore mother cell (a phenomenon called diplospory, e.g. Areva tomentosa). In such embryo sacs, the egg and all other cells are diploid (unlike a normal egg that is haploid). The diploid egg develops parthenogenetically without the act of fertilization into an embryo. It has been observed that stimulus of pollination is often required for parthenogenesis.
(ii) Non-recurrent Agamospermy:
Here the megaspore moter cell divides meiotically forming haploid embryo sac. The embryo is formed from such haploid egg or any other haploid cell of the embryo sac without fertilization. The plants formed from such embryos are mostly sterile, e.g., Zea mays, Datura.
(iii) Adventive Embryony (Adventive Polyembryony):
Here, the embryos develop directly from the diploid cells of nucellus or integuments of the ovule and not from a fertilized egg. In such cases, the embryos formed from fertilized eggs either degenerate or compete with the adventive embryos. Usually, adventive embryony results in the formation of more than one embryo in a seed. However, like parthenogenesis, stimulus of pollination is required for the formation of adventive embryos.