324 



GYMNOSPERMS 



Fig. 321. — Taxus canadensis: fe- 

 male gametophytc cellular through- 

 out, but before the appearance of 

 archegonia. The upper gamelophyte 

 is in the free nuclear stage, and there 

 is an abortive gametophyte between 

 them, at the right; X 286. — After 

 DUPLER.''* 



closure, two cell walls are in contact. 

 This is doubtless the reason why 

 many female gametophytes split so 

 easily in the middle. 



In Taxus,^''^ the first walls, perpen- 

 dicular to the megaspore membrane, 

 extend to the middle of the mega- 

 spore cavity, so that the cells are like 

 long tubes, which have been called 

 "alveoli." In nearly all cases the 

 walls on the centripetal ends of the 

 alveoli form late. Subsequent divi- 

 sions divide each of the alveoli into 

 a large number of cells. In Taxus, 

 the gametophyte is cellular through- 

 out before any archegonial initials 

 appear; this is true also of Torrcya 

 and others (fig. 321). 



There are long alveoli in Micro- 

 cachrys tetragona, but here, judging 

 from Lawson's figures,^^" the cen- 

 tripetal end of the alveolus has a 

 wall from the start. 



In general, the development of the 

 female gametophyte is much as in 

 the angiosperms, except that there 

 are none which are cellular from the 

 beginning, as in the long, narrow 

 embryo sacs of Ccratophyllum sub- 

 mersum and Monotropa Iiypopitys. 



It sometimes happens that more 

 than one megaspore germinates. 

 This is rather common in Taxus 

 canadensis, where more than one 

 megaspore mother-cell occurs fre- 

 quently. All four megaspores of a 

 tetrad may germinate, as well as one 



