Nichols, A morphological study of Jimiperus communis var. depressa. 203 



develop until the following season. The earliest stages to be 

 described are from material collected February ll th and kept in- 

 doors for two days. The differentiation of the tapetum and inner 

 layer of the sporangium wall from the sporogenous complex takes 

 place during the first week in April. A prolonged period of growth 

 then follows, accompanied by cell division, culminating abont May 1 st 

 in the formation of the microspore mother cells. 



During the winter the lower part of the sporophyll is com- 

 posecl of meristematic cells of nniform size and structure, while 

 the upper portion is already occupiecl by large vacuolate cells con- 

 taining numerous Compound starch grains. The latter are conspicuous 

 in many of the vegetative cells of the sporophyll until shortly 

 before pollination. The archesporium first becomes recognizable 

 as a plate of radially elongated hypodermal cells, four to six in 

 number when viewed in longitudinal section (fig. 5). Structurally 

 there is little to distinguish them from the adjacent sterile cells 

 save their somewhat denser cytoplasm. This layer soon divides 

 by periclinal walls (fig. 6), and the archesporium continues to in- 

 crease in size by the growth ancl division of its cells without any 

 differentiation becoming apparent. By the time the stage shown 

 in fig. 7 is reached the archesporium is without difficulty distin- 

 guishable from the vegetative tissue of the sporophyll through the 

 denser contents of its cells, while the base of the sporophyll has 

 begun to bulge slightly in the region where the microsporangium 

 is being developed. 



After a considerable mass of cells has been formed those 

 of the outer layer divide, and two layers of tabular-shaped 

 cells are cut off, which completely enclose a central mass of cells 

 (fig. 8). The latter may be termed the primary sporogenous cells, 

 since after a period of further growth and division they give rise 

 to the spore mother cells. The inner of the two enveloping layers 

 becomes the tapetum, while the outer layer develops into what 

 is usually termed the inner layer of the sporangium wall. This 

 latter layer, if viewed in a not quite mature sporangium, does 

 appear to be a part of the wall, since, after the breaking down 

 of their contents, the crushed cells form a thin layer which is 

 closely appressed to the outer layer of the wall (figs. 11, 12). 

 Morphologically, however, it is more closely correlated to the ta- 

 petum, both tissues being derived in the manner described above 

 from the archesporium, while the outer layer of the wall has quite 

 a different origin, as will be seen presently. Moreover the sub- 

 sequent history of this layer shows that physiologically also it is 

 a sort of accessory tapetum, contributing nourishment to the de- 

 veloping spores and eventually disappearing entirely. An analogous 

 case has been noted in Stangeria (Lang 1897) where the inner 

 layers of the wall are disorganized during the growth of the micro- 

 sporangium, and similar conditions are found in other Gymnosperms. 



Soon after the differentiation of the tapetum and the inner 

 wall layer from the sporogenous cells the majority of the sterile 

 cells in the lower part of the sporophyll become vacuolate. Certain 



