LIFE HISTORY OF PINUS 43 



pushed against the dorsal side of the spore-wall, not a vestige 

 of its cytoplasm is left, and the nucleus has become greatly 

 flattened, although there is still a faint suggestion of its former 

 reticular character (fig. 59). When the telophase of the divi- 

 sion is reached this nucleus has lost all traces of its former 

 structure and persists only as a deeply staining, linear body 

 lying against the spore-wall (fig. 60). During the following 

 division it becomes scarcely more than a line so that it is fre- 

 quently detected with difficulty. Coulter and Chamberlain 

 ('01) figure this cell in Pinus Laricio as still projecting into the 

 cytoplasm of the apical cell when that cell is in the telophase of 

 the second division, but I have never found it in such a state of 

 preservation at so late a date. The second prothallial cell is 

 invariably smaller than the first, and during the third mitosis of 

 the apical cell, which follows immediately the formation of the 

 second prothallial cell, it exactly repeats the history of the first 

 cell (figs. 61-63). 



The partial, broad, innermost wall, described in connection 

 with the development of the microspore, persists throughout the 

 entire history of the pollen-grain, and a comparatively broad 

 wall, continuous with it and having exactly the same staining 

 capacity, invests both the first and second prothallial cells as 

 shown in figs. 57-63. The presence of the remnants of the 

 prothallial cells imbedded apparently in the inner wall of the 

 mature pollen-grain (fig. 63) was very perplexing before the 

 history of these cells was studied. But in tracing their develop- 

 ment it is clearly demonstrated that the remnant of each cell is 

 pushed back against the wall of the spore and remains perma- 

 nently covered on its outer side by its own wall. That the 

 remains of these cells come to lie nearer the intine than when 

 first formed would again suggest the somewhat plastic nature 

 of the partial or incomplete membrane against which the pro- 

 thallial cells are pressed (figs. 57-64). These observations con- 

 firm the statement of Strasburger, Noll, Schenck and Schimper 

 ('97) that the two prothallial cells formed in the pollen-grain of 

 the Gymnosperms are invested with cellulose-walls. Coulter 

 and Chamberlain ('01) make no mention of the formation of 

 walls in connection with the development of these cells in Pinus 



