1907] SMITH—TRUNK AND MICROSPORANGIUM OF CYCADS 197 . 
this breaking down of cells is combined in Angiopteris with the 
regular mechanical arrangement for dehiscence. 
In Zamia the stomata are situated in about the same region of 
the sporangium as in Stangeria. They are especially numerous 
among the thinner cells of the lower part of the sporangium, and the 
guard cells in cross-section are only about one-half as high as the 
other cells. The walls of the guard cells and of the subsidiary cells 
are strongly thickened, and the whole apparatus resembles that 
found among xerophytic angiosperms. The stomata of Ceratozamia 
are not so deeply sunken; but in Encephalartos they are more deeply 
placed than in Zamia (figs. 39, 41). 
In Zamia and Ceratozamia the tapetum (some of it at least) is 
derived from the sporogenous tissue. The blocking out of the cells 
and the irregular width of the layer, especially toward the base of the 
sporangium, suggests this, and in many cases can only be explained 
by the fact that there is a distinct transformation of sporogenous 
cells into tapetum. In many sections it is hard to distinguish the 
tapetum, but in the stage shown in fig. 42 the nuclei are smaller and a 
little more elongated, and the chromatin stains more deeply. At 
the base of the sporangium the irregularity in the width of the tapetum 
is more striking. Usually there are one or two layers all around the 
sporogenous mass; but fig. 43 shows four or five layers, dipping into 
the sporogenous tissue. The nuclei of the tapetal cells become divided 
and spindles (fig. 42) are occasionally seen, so that although there 
may be fragmentation, as LANG (16) states, there is also regular 
mitosis. The tapetum is a distinct layer up to the tetrad stage, when 
the walls disappear, and the mass of nutritive substance lines the 
sporangial cavity. 
There is some sterilization of sporogenous tissue, for occasionally 
regions of tissue in the center of the sporangium or at the base, with 
extensions into the center, become sterilized, and, as has been men- 
tioned, the cells near the base often appear to function as tapetal cells. 
The division of the spore mother cell was observed in Ceratozamia 
mexicana (fig. 44). The spirem in the nucleus of the mother cell is 
large and clear, the chromosomes are thick and short, so that the 
chance for counting is unusually good. A count was made in 50 
nuclei, resulting in 12 chromosomes in 46 cases, II in 3 cases, and 13 
