322 



GYMNOSPERMS 



mother-cell, the lower of the two resulting cells is likely to divide 

 first. Although the division in the two cells is said to be simulta- 

 neous, and the two mitotic figures are seen at the same time, the lower 



figure is practically always a little 

 more advanced, and its daughter- 

 nuclei reach the resting condition, 

 while those of the upper mitosis 

 are still in the late telophase. 

 Thus the lower two, and especially 

 the lowest one, get into the resting, 

 or, rather, the working, condition 

 earlier than the upper pair. An ex- 

 cellent example is seen in Land's 

 drawing of Ephedra, in which the 

 lowest megaspore is enlarging, the 

 next above is beginning to disor- 

 ganize, while the mitosis in the 

 upper cell of the first "pair" is still 

 in the late telophase (fig. 359/!). 

 The reason for the more rapid de- 

 velopment of the lowest cell is 

 probably because the nutrition 

 comes principally from beneath. 

 In gymnosperms, almost invaria- 

 bly , and nearly as frequently in an- 

 giosperms, the lowest megaspore 

 is the one which germinates and de- 

 velops into the adult gametophytc. 

 Free nuclear period. — In all co- 

 nifers there is a period of free nu- 

 clear division before any cell walls 

 are formed. Even as early as the 

 close of the first division, a large vacuole may appear between the 

 two daughter-nuclei. With succeeding divisions the outUne of the 

 megaspore increases immensely, and the central vacuole, filled with 

 a transparent fluid, presses the protoplasm outward so that it forms 

 a thin layer containing the free nuclei (lig. 320). Mitoses through- 



FiG. 320. — Pinus strobus: free nuclear 

 stage in female gametophyte. The proto- 

 plasm, with its nuclei, is pressed outward 

 by the large central vacuole. The gameto- 

 phyte is surrounded by a jacket of spongy 

 tissue; X62. — .After Dr. Margaret Fer- 

 guson. ■»* 



