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 sporogenons mass, 
and these eventnally give rise to the one-layered wall of the mature 
sporanginm. At first these cells are scarcely distinguishable from 
those of the archesporinm, but they may be recognized by their 
Position and prismatic shape. This deriyation 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, ronnds 
out, and becomes easily visible to the naked eye. The sporogenons 
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 läyer 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 /a in diameter — and their 
nuclei rarely exceed 12 ; u. 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 Linus have been the favorite objects 
among the conifers, for in these genera the nuclei of the pollen 
mother cells are from 25 to 35 t a in diameter. 
The nucleus of the mother cell (fig. 15) possesses a well de- 
fined membrane and a reticulum consisting of deeplv 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 
