5° BOTANICAL GAZETTE [JULY 
In the development of the stamen the enlargement of the tip, fore- 
shadowing the formation of sporogenous tissue, occurs just about 
the time the carpels appear. The hypodermal layer gives rise to a — 
primary parietal layer, and another homologizing with what is ordi- 
narily the primary sporogenous layer (fig. 4). The former divides 
once; the latter also divides, forming the tapetum and primary ~ 
sporogenous cells (fig. 5). This has been observed in a few plants — 
only (r), the tapetum usually arising from the primary parietal layer. 
The primary sporogenous cells elongate as they do in Asclepias 
(fig. 6), but divide into a mass of mother cells, thus reinforcing the 
presumption that in Ascelpias this stage is simply omitted. The 
pollen is in the mother cell stage when the ovules appear. The 
rounded mother cells do not always divide simultaneously. Division 
of the two daughter cells is simultaneous or nearly so (fig. 8), and 
almost so in all the daughter cells of a sporangium; but not in different — 
stamens of the same flower. Sometimes one daughter cell fails to 
divide, and three microspores instead of a tetrad is the result (fig. 9). 
Occasionally some of the pollen grains near the tip of the sporangium 
disintegrate after tetrad division, probably serving as nourishment 
for the others. The whole tapetum also disintegrates soon after 
tetrad division. 
The microspores remain in tetrads in maturity, and their arrange- j 
ment with relation to each other is various. In fact, one can find — 
all stages grading from the bilateral to the tetrahedral arrangement. 
Fig. 10 evidently resulted from the spindles in the second division 
being somewhat at right angles in the same plane, and is like a group- 
ing found by WILLE (2) in Orchis mascula. Usually the four spores 
are in the same plane, but their arrangement with regard to each 
other varies; in fig. 11 four pollen grains meet at a point on each 
side of the group; in other cases there are four on one side and three 
on the other; in still others only three meet at a point on either side. 
The pollen grains in a tetrad are often plainly unequal in size. I 
the prevailing dicotyl grouping—tetrahedral. Figs. 11, 12, 13 
are three members of a series grading from the bilateral to the tetra~ 
