204 Nichols, A morphological study of Juniperus communis var. depressa. 



epidermal cells, however, which are in direct contact with the wall 

 layer remain in an embryonic condition, dividing by anticlinal walls 

 to keep pace with. the now rapidly enlarging sporogenous mass, 

 and these eventually give rise to the one-layered wall of the mature 

 sporangium. At first these cells are scarcely distinguishable from 

 those of the archesporium, but they may be recognized by their 

 Position and prismatic shape. This derivation of all or a part of 

 the sporangium wall directly from the epidermal layer is of uni- 

 versal occurence among gymnosperms and is in marked contrast 

 to the conditions found among the Angiosperms where the epi- 

 dermis plays no important part in the formation of the wall of the 

 mature pollen sac. 



The microsporangium now increases enormously in size, rounds 

 out, and becomes easily visible to the naked eye. The sporogenous 

 cells continue to grow and divide actively until shortly before 

 synapsis, but cell division in the two wall layers soon stops. The 

 cells of the outer wall layer for a time enlarge and develop large 

 vacuoles which become filled with some amorphous substance, 

 presumably of a resinous character, while the cells of the inner 

 wall layer cease to grow and, in consequence of the pressure from 

 within the sporangium, become stretched and flattened, and their 

 nuclei and cytoplasm disorganize, so that by the conclusion of the 

 reduction division the cells are usually crushed and structureless. 



The cells of the tapetum continue to divide by anticlinal 

 walls until shortly before the maturity of the mother cells. Growth 

 still goes on in them after the cessation of division, vacuoles appear, 

 and they may be distinguished from the sporogenous cells by their 

 paler color and smaller nuclei. The tapetum attains its maximum 

 development during the cell divisions which precede spore formation 

 and disorganizes rapidly after the formation of the pollen, disappearing 

 entirely before pollination (figs. 9 — 12). 



During the brief period of rest which precedes synapsis 

 (fig. 9), the mother cell complex appears as a compact mass of 

 thin-walled, polyhedral cells with large nuclei and dense proto- 

 plasmic contents. As noted by Noren (1907), the mother cells 

 are comparatively small — about 22 ju in diameter — and their 

 nuclei rarely exceed 12 ju. It is therefore difficult and sometimes 

 impossible to follow with certainty many of the complex nuclear 

 phenomena which characterize the heterotypic division. For research 

 along these lines Larix and Pinus have been the favorite objects 

 among the conifers, for in these genera the nuclei of the pollen 

 mother cells are from 25 to 35 ^ in diameter. 



The nucleus of the mother cell (fig. 15) possesses a well de- 

 fined membrane and a reticulum consisting of deeply staining, 

 knot-like masses connected by inconspicuous, lightly staining threads, 

 the whole forming an irregularly anastomosing network. Concerning 

 the nucleus at this time Noren (1907, p. 8) writes: "Wir sehen 

 die Chromatinkörner zu sehr kleinen, scharf begrenzten Körpern 

 gruppiert, die meistens paarweise auf kurzen Lininfäden sitzend 

 hauptsächlich in der Peripherie des Kerns gelegen sind." If such 



