LIFE HISTORY OF PINUS 83 



18-20). The persistence of the potential megaspores in Larix 

 at this time is also in very striking contrast to Pimcs, where the 

 other cells of the axial row have become entirely absorbed before 

 the germination of the macrospore occurs (figs. 147-149). 



The third division of the macrospore-mother-cell, or the first 

 division of the macrospore-nucleus, takes place during the very 

 last of July or the first of August in all the species studied, 

 and is of the ordinary or typic method. It differs from the 

 mitoses occurring in the vegetative tissue of the sporophyte 

 only in presenting the one-half number of chromosomes (fig. 

 150). The daughter-nuclei may remain at one side of the pro- 

 thallial cavity, but more frequently they pass to opposite sides 

 as in the development of the embryo-sac in Angiosperms (fig. 

 151). The second mitosis follows rather quickly, and is already 

 completed in Pinus Strobus on August 4 (fig. 152). Nuclear 

 divisions follow until several free nuclei have been formed. 

 The observations of Strasburger ('79), and of all later students 

 of the Gymnosperms, upon the simultaneous division of the free 

 nuclei of the endosperm have been confirmed. On October 

 12, 1898, sixteen nuclei were observed in the cytoplasmic layer, 

 all being in the spireme stage of division. On October 15 of 

 the same year sixteen nuclei, all presenting the equatorial plate- 

 stage of mitosis were found in the cytoplasm of the prothallium, 

 (figs. 153-155). The karyokinetic figure is sharply bipolar, 

 each pole ends in a slight condensation of the cytoplasm, and 

 the chromosomes are clearly of the reduced number. 



I find no evidence that any further divisions occur during 

 the first period of growth and it is probable that the thirty-two 

 nuclei which result from the division just described pass into 

 the resting stage and remain inactive during the winter. But I 

 have not examined a sufficiently large number of preparations 

 with this point in mind to affirm that the prothallium of Pinus 

 invariably enters upon its long period of rest in the thirty- two 

 nucleated stage. The number may not be fixed even in the 

 same species, but it is certain that it is never large. The pro- 

 thallium, therefore, at the close of its first season of growth is 

 a spherical body composed of an ectal layer of cytoplasm in 

 which are imbedded, in many instances at least, thirty-two free 



