372 APOGAMY, ETC. 



leaves is distinguishable, in the Castor Oil the plumule is merely 

 a peg-like structure, whilst in the Lesser Celandine (Ficaria) the 

 entire embryo is an undifferentiated mass of cells (see also 

 Fig. 217, H), a condition likewise encountered in the embryos of 

 most parasites. In some plants, moreover, there is no resting 

 period (e.g. in the tropical Mangroves), whilst in many trees the 

 seeds germinate most readily if sown immediately on reaching 

 maturity. 



Exceptions to the normal sequence of events described in 

 the foregoing pages are by no means uncommon. Apogamy 

 (cf. p. 305), for example, has been recorded in quite a large 

 number of Compositae (e.g. Dandelion, Hawkweed), as well as 

 in some species of Lady's Mantle (Alchemilla). In such cases 

 the reduction division does not appear to occur, and the 

 embryo arises from an unfertilised cell of the embryo sac having 

 the normal number of chromosomes. Such apogamy is of course 

 akin to vegetative propagation, but gains the advantages afforded 

 by the mechanism for seed-dispersal. Despite the non-occurrence 

 of a sexual process in such apogamous forms, pollination some- 

 times appears to furnish a necessary stimulus for embryo- 

 formation. More rarely it is an ordinary cell of the nucellus 

 that divides to form the embryo (e.g. Orange, Citrus), a case 

 analogous to the apospory described among Ferns (p. 305). In 

 the Hawkweeds all three conditions apogamy, apospory, and 

 normal fertilisation have been observed. 



The occasional presence of more than one embryo within a 

 seed may be due to several causes. Sometimes more than one 

 member of the tetrad, formed by the megaspore mother-cell, 

 develops into an embryo sac, so that several embryos are present 

 from the first. But more frequently accessory embryos arise by 

 vegetative budding from the proembryo (Fig. 217, F). Orange 

 pips frequently contain several embryos, of which one is the 

 outcome of a sexual fusion*, whilst the others are derived from 

 nucellar cells which are presumably stimulated to growth as a 

 result of fertilisation (Fig. 217, J). 



The general course of the life-history in Angiosperms is 

 obviously very similar to that of Gymnosperms. In both cases 

 the young embryo lives, as a parasite, within the ovule, which 

 forms a protective envelope around it until the time of germina- 



