174 EMBRYOGENESIS IN PLANTS 



life on land, of parts already present. . . . The biological probability 

 of the several steps disclosed by this comparison is such as to justify 

 their acceptance as evolutionary history.'^ In a later Chapter we shall 

 have occasion to consider the homologies that exist between the embryo- 

 sac in flowering plants and that in the gymnosperms. Lastly, it may be 

 noted that, in tiie gymnosperms, several archegonia may be fertilised, 

 and that polyembryony, a feature already noted in some pteridophytes, 

 is of very general occurrence. 



In passing it may be noted that some investigators have stressed 

 that heterospory is not a necessary postulate in a theory of origin of 

 seed habit but rather heterothally (Thomson, 1927; Doyle, 1953). 

 Doyle, indeed, suggests that the terms androspore and gynospore should 

 be used instead of microspore and megaspore. There is increasing 

 palaeontological evidence, however, of the incidence of heterospory in 

 ancient pteridophytes. 



Outline of Embryogenesis in Pinus. The details of embryogenesis in 

 different gymnosperm groups are variable; they may also be somewhat 

 complicated. In Pinus, to give a preliminary and simplified indication 

 of development in one type, the zygote nucleus within the archegonium 

 divides at once, and then again. The four nuclei so formed move to 

 the basal end of the archegonium and lie in a single plane. Fig. 37. 

 Further nuclear divisions follow so that four tiers of four nuclei are 

 formed, each nucleus being subsequently separated from its neighbours 

 by thin cell walls. The basal cells, i.e. those lying at the base of the 

 archegonium, are the embryonic cells; the cells of the adjacent tier 

 elongate to form the suspensor, or suspensors. In Pinus and some 

 related genera, this cell complex at the base of the archegonium may 

 cohere to form one single large embryo, or each linear series of cells 

 may separate, with the result that four separate embryos are formed 

 side by side. Although at first several embryos may be present, sooner 

 or later one embryo in each ovule secures the ascendancy and the others 

 become inhibited and absorbed. The enlarging embryo is thrust by its 

 suspensor down into the prothallial tissue and there it matures into 

 a germ with an apical growing point, numerous cotyledons, a hypocotyl 

 and a massive root. The prothallus, which persists and functions as a 

 nutritive 'endosperm,' enlarges and crushes the nucellus against the 

 hardening integument which becomes the seed coat. (For a fuller 

 account of the Pinaceae, see p. 190.) 



Classification. The taxonomic and phylogenetic seriation of the 

 gymnosperms is difficult and many important problems still await 

 solution. The evidence suggests that the gymnosperms originated in 

 ancient pteridophyte ancestors as two main lines of descent, the 



^ The pollen tube is, of course, also very significant in the evolution of seed plants. 



