436 Lang. — Studies in the Development and 
sub-epidermal position of the sporogenous cells 1 would tend 
to diminish the importance to be attached to this difference 
between Marattiaceae and Cycadaceae. It is to be noted 
that, though the stratification of the apical meristem was 
imperfect in the Cycads investigated, the stratification is 
perfectly definite in the young sporophyll, the epidermis 
never undergoing periclinal divisions. 
The general agreement between the sporangia of these 
groups lends weight to this mode of regarding the question. 
The resemblance in general form and appearance between 
the sporangia of Cycads and Marattiaceae has been noted 
especially by Braun and Warming. There are two points, 
however, which deserve further notice, the origin of the 
tapetum, and the mode of dehiscence of the sporangium. 
In the Marattiaceae the tapetum arises normally from the 
layer of the wall adjoining the sporogenous cells, and, on 
theoretical grounds, a similar origin might have been expected 
in the bulky sporangia of St anger ia. The case has been 
shown to be more complex, however. Three layers of cells 
around the mother-cells of the pollen-grains are found to 
have undergone special changes. The innermost layer is 
derived from the sporogenous mass, while the other two 
layers arise from the cells of the wall adjoining it. Since 
only the inner layer takes on tapetal characters, the tapetum 
has been described as arising from sporogenous cells. But 
the layers outside, which doubtless contribute to the nutrition 
of the spore-mother-cells, may possibly represent a tapetum 
derived from cells surrounding the sporogenous group, an 
additional tapetal layer having arisen from the latter. This 
explanation becomes more likely when it is borne in mind 
that other cells throughout the sporogenous mass lose the 
function of spore-production, and their substance aids in 
the nourishment of the rest. The difference in appearance 
of the tapetum from the isolated sterilized cells may be 
connected with the additional function it performs in virtue 
1 Bower, Studies in the Morphology 'of Spore-producing Members : Ophio- 
glossaceae, p. 7. 
