COMPARATIVE MORPHOLOGY OF GYMNOSPERMS 



161 



which passes over directly into a mother cell, and that the pecul- 

 iar differentiation and breaking down of a zone of tissue about 

 it has led to the impression of a sporogenous mass. 



In some cases where there is more than one mother cell, two 

 or more megaspores begin to develop, and in Sequoia the sterile 

 mega spores are prominent and persistent, but as a rule one soon 

 becomes dominant at the expense of the rest and of the adjacent 

 nucellar tissue. 



The mother cell increases in size in the usual way, and early 

 in its history begins to germinate. The sequence of events 

 seems to be very uniform, being free nuclear 

 division (usually about eight simultaneous 

 divisions), parietal placing of free nuclei in 

 a cytoplasmic layer, organization of pari- 

 etal tissue, and gradual filling of the embryo 

 sac by centripetal growth. The cases of 

 Tumboa and Gneium are peculiar in that 

 there is a distinct polarity in the tissues of 

 the sac, and an appearance of reproductive 

 cells in a sac not filled with tissue. In Turn- 

 boa the micropylar region of the sac con- 

 tains cells but loosely aggregated when the 

 reproductive cells appear, while in Gnetum 

 the micropylar end of the sac, and in some 

 species the whole sac, contains only free 

 nuclei when fertilization occurs. Indica- 

 tions of the same tendency in other forms 

 are not lacking. In Sequoia it is reported 

 that while cell formation is taking place 

 at the two extremities of the sac, free nuclear division con- 

 tinues in the middle region, the sac thus being distinctly divided 

 into its reproductive and nutritive regions. 



In the development of the archegonia, which are variable in 

 number and in distribution, there are some features that seem 

 peculiar to the group. The neck canal cell 'series, which is by 

 no means prominent in the Pteridophytes, is not represented at 

 all in the Gymnosperms; and the ventral canal cell, cut off 

 immediately before fertilization, is usually very minute and 

 ephemeral. Probably the most noteworthy fact in connec- 

 tion with the archegonium is the organization of a sheath of 



FIG. 106. Pinus Lari- 

 cio, showing the four 

 potential megaspores 

 derived from the 

 mother cell, x 666. 



