1432 A TEXTBOOK OF THEORETICAL BOTANY 



behaviour of the polar nuclei is inconsistent with anything known of the 

 ventral canal cells in Gymnosperms. He denies the existence of any trace 

 of an archegonium in the angiospermic embryo sac and suggests comparison 

 with the embryo sac of Gnetiim, in which also the archegonia have dis- 

 appeared. This is not to suppose a derivation from the Gnetales but that 

 there has been a parallel development in the evolution of the Angiosperms. 



According to this view, gametogenesis is limited to the differentiation 

 of one nucleus as the oosphere. The stages of development between spore 

 formation and gametogenesis, involving, in the normal case, three nuclear 

 divisions and culminating in wall formation after the last division, are 

 regarded as somatic development and include vacuolation of the sac and 

 polarization of its nuclei. Dispone development (Scilla) allows for the 

 omission of one somatic division, while tetrasporic development {Adoxa) 

 allows for the omission of two somatic divisions, to give finally a normal 

 eight-nucleate sac. 



Before leaving the subject of the embryo sac, some reference should be 

 made to the question of whether the term " megaspore " is properly applied 

 in the description of its development. The evolution of the seed plants from 

 heterosporous ancestors has been so widely and for so long assumed that the 

 expression of a doubt may occasion some surprise. Thomson in 1927 was 

 the first to voice his scepticism and it has recently been revived by Doyle. 

 Both base their views on comparative measurements of the male and female 

 spores in Gymnosperms, which show that in the majority of cases the so- 

 called microspore is actually the larger of the two, whereas in heterosporous 

 Pteridophyta like Selaginella or Marsilea, the megaspores are very much 

 larger. Doyle therefore questions the propriety of using the term mega- 

 spore among seed plants and substitutes "gynospore", a non-committal 

 name. 



It must be pointed out that under the conditions obtaining in the ovule 

 it is necessary for purposes of comparison to fix somewhat arbitrarily on a 

 certain stage, namely the beginning of vacuolization, when the female spore 

 is taken to be mature and starting on the development of the prothallus. 

 This may be justified, but when we regard conditions in the Pteridophyta 

 we see there that the megaspore grows to its full size first and develops a 

 prothallus afterwards, without further increase of size. If therefore measure- 

 ments are made in the Gymnosperms taking for comparison the stage at 

 which the prothallus has been formed, which is more nearly equivalent to 

 the mature spore in Selaginella, then there is an equally great advantage of 

 size in favour of the female spore. It is very desirable that further investiga- 

 tions of the question should be made among the Angiosperms. 



FERTILIZATION 



We now arrive at the crux of the whole elaborate process of sexual repro- 

 duction, which is the meeting and fusing of a male gamete nucleus and a 

 female gamete nucleus. Owing to the seclusion and protection of the female 



