132 



MORPHOLOGY OF GYMNOSPERMS 



3. The gametophytes 



THE FEMALE GAMETOPHYTE 



The megaspore is the first cell of the female gametophyte. It 

 germinates immediately, a prolonged period of free nuclear division 



being accompanied by a 

 much greater increase in 

 the size of the embryo 

 sac than in the quantity 

 of cytoplasm, resulting in 

 the formation of a large 

 central vacuole which 

 presses the cytoplasm 

 with its nuclei against 

 the megaspore mem- 

 brane. Although cell- 

 formation has not been 

 studied among cycads, it 

 is evident that it begins 

 at the periphery of the 

 embryo sac and advances 

 toward the center, as has 

 been described for other 

 gymnosperms (figs. 132- 

 134) . From the figure it 

 will be seen that cell-divi- 

 sion is more vigorous at 

 the periphery. Even 

 after the gametophyte 

 has become cellular 

 throughout, sugar is the 

 principal food content of 

 the cell; but as the ovule 

 approaches its full size, 

 starch becomes increas- 

 ingly abundant and in the mature seed it occupies almost the entire 

 cell. The cells of the outermost layer are sharply differentiated 

 from the rest, not only by their smaller size, but also by their almost 



Figs. 132-134. — Dioon edule: stages in the 

 development of the endosperm; fig. 132, small 

 portion of periphery of endosperm some time after it 

 has become cellular throughout; fig. 133, still later; 

 the two rows (a) have come from a single row (like 

 a) of the preceding figure; fig. 134, mature endo- 

 sperm, showing thick megaspore membrane at the 

 left, the outer row of cells with little or no starch, 

 the next row with small starch grains, and the rest 

 with large starch grains; the evenly shaded cell 

 contains tannin; allX88. — ^After Chamberlain (46). 



