THE FEMALE GAMETOPHYTE 



75 



logue of the tetrad formed by the microspore mother-cell. The 

 most recent suggestion as to the nature of the embryo-sac is that 

 made by Atkinson, 101 who claims that in the ovule there do not 

 exist spores " in the sense in which they are represented in the 

 Pteridophytes, or in the microspores of the Spermatophytes," 

 but that the angiospermous embryo-sac arises directly from 

 nucellar tissue without the intervention of spores. As spores 

 are not needed for distribution they are " cut out of the cycle 

 of development, and the embryo-sac or gametophyte arises 

 directly from the tissue of the sporophyte." 



In our own judgment it seems clear that the cells in question 

 are morphologically megaspores, and if so it would follow that 

 the natural tendency of the megaspore mother-cell is to form a 

 tetrad. The fact that the spores form a row may be due to the 



C 



Fig. Z\.—Hyacinthu& orientalis. Abnormal germination of microspores in petaloid 

 anthers. A, the microspore has formed a sac-like tube showing definite polarity; 

 B, a short pollen-tube resembling an embryo-sac at the third nuclear division ; the 

 heavy line below represents the thick wall of the pollen-grain; C, a later stage, 

 showing four nuclei at each end of the sac-like pollen-tube.— After Nemec." 



pressure of the surrounding tissue, there being no opportunity 

 for early isolation and rounding off as in microsporangia. It 

 may be of interest to note that sometimes after the first divi- 

 sion of the mother-cell the outer daughter-cell divides by an 

 anticlinal rather than a periclinal wall, as observed in Butomus 



