LIFE HISTORY OF PINUS 85 



At some time during the latter part of May in Pinus Strobus 

 and about the middle of the month in the other species free 

 nuclear division ceases and cell-walls are developed between the 

 nuclei. The development of the prothallium from this point 

 on was studied by Sokolowa ('80), and her observations have in 

 general been confirmed by all more recent writers, with the 

 exception of Jager ('99) in Taxus. I find the development of 

 cell-walls in the prothallium of Pinus to agree perfectly in its 

 early stages with that described by Sokolowa. Walls are 

 formed perpendicular to the wall lining the prothallial cavity, 

 thus each nucleus with its proper portion of the cytoplasm is 

 separated from all the other nuclei. No wall is laid down on 

 the inner sides of these cells, so that in radial section the cells 

 appear as uncovered boxes, the opening extending towards the 

 center of the prothallial cavity. In surface view the cells are 

 more or less isodiametric, polygonal in outline and very uniform 

 in size. A layer of densely reticulated cytoplasm surrounds 

 each nucleus, and delicate strands radiate from it to the ectal 

 layer of cytoplasm, thus giving a very different aspect to the 

 cytoplasm than it had prior to the development of cell walls 

 (figs. 160 and 161). Jager described the presence of walls on 

 the inner face of these cells in Taxus when the cells were first 

 organized, but other students have not confirmed his observa- 

 tions. 



According to Sokolowa these cells grew inwards forming long 

 open tubes which extended to the center without division, a wall 

 was then formed at the inner end and the cells became divided 

 by cross walls. To these long cells the name alveoli was 

 applied. Only those from the sides extended clear to the center 

 before being closed, those from the extremities becoming more 

 or less wedge-shaped. Jaccard ('94) notes that inJSphedra some 

 of the alveoli may divide before reaching the center, but many 

 do not, while Arnoldi ('99 and '01) finds that no division occurs 

 in Sequoia until after the alveoli have met at the center and their 

 ends have become closed by walls. The development sub- 

 sequent to the formation of the open cells varies considerably 

 in Pinus from that described by these writers for other Gymno- 

 sperms. No cell has ever been observed to extend from the 



