Bot.— Vol. I.] CANNON— A VENA FATUA. 337 



nuclei form a larger portion of the cell contents than is 

 indicated in the figure (fig. 12). Koernicke does not men- 

 tion a division of the generative nuclei in the pollen grain 

 of wheat, and this would be an apparent exception to the 

 statement by Strasburger (1884), that the division of the 

 generative nuclei is a constant character of the grasses. 



The generative nuclei in the pollen grain of Avena 

 occupy various positions with relation to the vegetative 

 nucleus, but are always parallel to each other. The vegeta- 

 tive nucleus is oblong and as large as both the other two 

 nuclei combined. The development of the pollen grain 

 was not followed further. 



IV. The Embryo-Sac. 



The initial cell of the embryo-sac in the oat is of sub- 

 epidermal origin. It is the middle one of the epidermal 

 cells of the primordium, as is generally the case (Fischer), 

 and it is continuous with the axial plerome. At the time 

 when both of the integuments of the ovary are clearly dis- 

 tinguishable, the archesporial cell is easily made out by 

 reason of its relatively large size. At this stage it occupies 

 in the oat, as in the wheat (Koernicke), over one-third of 

 the nucellus, and is eight to ten times as large as the sur- 

 rounding cells. There is no apparent difference at this 

 time between the archesporial cell and those of the nucellus 

 in density of cell contents. 



When the archesporial cell is first recognizable the ovule 

 is atropous. Parallel with the development of the sister 

 cells and of the embryo-sac, the cells in the rachilla side of 

 the nucellus divide and grow more rapidly than those in the 

 opposite side, and push the micropylar end over so that 

 finally the micropyle occupies a position almost opposite 

 from that of the archesporial cell. During the revolution 

 of the ovule (macrosporangium) the ovary increases greatly 

 in size, and, from being somewhat flattened at first, it 

 becomes spherical and at last oblong. The size and posi- 

 tion of the ovule is intimately connected with the develop- 

 ment of the archesporial cell and the embryo-sac. 



