422 SCIENCE PROGRESS 



spread have been the species in which the triple endosperm- 

 producing fusion has been described that probably no botanist 

 would hesitate now to speak of this as the normal function 

 of the second male nucleus. 



The probable universality of the process, then, being admitted, 

 let us inquire into its possible history and significance. 



The ovum and the other nuclei in question are enclosed 

 within a membranous bag set in the heart of the ovule, and 

 known as the embryo sac. The embryo sac is undoubtedly a 

 retained and enclosed spore, never set free from the plant. Its 

 contents are therefore homologous with the products of germi- 

 nation of a fern spore, that is, to the little green prothallus 

 together with the reproductive organs that it bears. Now there 

 are other plants, of the same general rank as ferns, which 

 differ from them in that their spores produce prothallia of two 

 kinds, bearing one or other sexual organ. The embryo sac of 

 flowering plants thus more strictly corresponds with a spore 

 producing female prothallia. 



In the Gymnosperms, which form a group roughly inter- 

 mediate in complexity between the ferns and flowering plants, 

 the spore (= embryo sac) destined to produce female reproductive 

 cells is retained by the plant as in the Angiosperms, but the 

 ovum is produced in an organ which, although much reduced, is 

 plainly comparable to the archegonia of fern prothalli. Thus 

 the reproductive tissue is quite plainly marked off from the 

 vegetative. 



In the Angiosperms, on the other hand, reduction has taken 

 place to such an extent that the vegetative and reproductive 

 tissues within the spore — together represented by only eight 

 nuclei — show no obvious differentiation, and would-be theorists 

 must fall back upon the guides of "origin" and "destiny" so 

 much used by zoologists. 



Origin shows that one of the nuclei with which the second 

 male nucleus fuses is sister nucleus to the ovum — i.e. produced 

 by the same nuclear division — and therefore presumably a 

 potential ovum ; destiny shows that it behaves as an ovum in 

 fusing with a male nucleus, and were it not for the presence of 

 another nucleus — probably vegetative in character — making up 

 the much talked of triple fusion, we should have here a second 

 act of fertilisation within the embryo sac. Should we also have 

 a second embryo, and is the presence of the intrusive vegetative 



