248 BOTANICAL GAZETTE | APRIL 
secreted materials are rendered available by the total collapse of 
the cells. 
In many plants also, especially in those in which the tapetum 
undergoes complete disorganization, it is common for the tapetal 
cells to become multinucleate, the division of the nuclei being 
sometimes accomplished by karyokinesis, but mostly by amitosis. 
The cells of the tapetum of Isoetes, in this respect again agreeing 
with Lycopodium and Selaginella, are uniformly uninucleate. 
In almost every sporangium examined the number of layers 
of cells outside the fertile regions when they first become dis- 
tinct is three. In a very few cases there were four layers. As 
already shown, the innermost of these becomes tapetum. Of 
the other two layers, one, apparently the hypodermal, usually 
undergoes division, so that the wall region ultimately consists of 
three layers outside the tapetum. 
At the base of the sporangium, between it and the vascular 
bundle, are a few layers of cells which may be regarded as the 
inner wall of the sporangium. The exact origin of these I have 
not been able to make out. Whether, like the outer wall, they 
are derived from the sterilization of sporogenous tissue, oF 
whether they are derived from the tissues underlying the original 
archesporium, I cannot say. It is always difficult in all sporangia 
except the very youngest to define the exact inner limits. 
Between the vascular bundle and the three or four outer layers 
where growth and division are most actively carried on, there is 
a mass of small cells staining deeply. Such a section as fig. 38 
makes it probable that all the tissues between the parenchyma- 
tous sheath of the xylem and the outside arises from the sporan- 
gium Andage, and that therefore the inner wall arises also by 
sterilization. 
: The formation of the microspores in Isoetes takes place in 
much the same Way as in other vascular plants. After the 
fertile regions have ceased their cell divisions, the cells and 
their nuclei pass through a period of rest and enlargement. 
The nuclei especially increase in size and become rich in chro- 
matin. At the same time the cytoplasm remains dense and never 
