20 BOTANICAL GAZETTE [JULY 
reduced to a single cell (the megaspore mother-cell), the tissue 
immediately around this cell must be considered asa tapetum, 
and we have seen that it is probably by the division of this 
tissue that the nourishing layer is formed. This cannot as yet 
be positively asserted for the later stages, as all steps have not 
been followed. The only other interpretation that seems possible 
is that the cells immediately surrounding the megaspore represent 
an originally archesporial tissue which has given up its function 
of spore-production and taken up the new réle of nourishing the 
young plant within. Of these two interpretations I consider the 
first as much the more likely. 
It is well known that the tapetum in the megasporangium of 
Selaginella persists intact until some time after the sprouting of 
the spores, which in this case are shed only after a considerable 
growth has occurred. Should the spores not be shed at all, and 
the tapetum continue still further its growth and function, we 
would have a condition paralleling that found in Taxodium. 
DEVELOPMENT OF THE PROTHALLIUM. 
We have left the megaspore to follow the surrounding tissue 
through its subsequent stages. The growth of the germinating 
megaspore is extremely slow during the first month after its 
formation, having reached on April 29 only about the 32-celled 
stage (fig. 47). Growth now becomes more rapid, and on 
May 6 it has reached the stage shown in fig. gg. About the 
beginning of June growth has stopped in the upper part and the 
formation of cell walls begins. ig. 25 shows astage just before 
the beginning of cell-formation. The pollen tube has already 
reached the megaspore, which now contains an enormous 
vacuole surrounded by a protoplasmic layer containing nuclei. 
In this case the protoplasmic layer has collapsed. 
The great size reached by the germinating megaspore before 
the formation of cellular tissue seems to distinguish Taxodium 
from other conifers thus far studied. The wall of the spore 
was found to be furnished with pits. Figs. 97 and 98 show 
those pits in surface view at a time shortly before the formation 
of a cellular prothallium. The manner of the formation of the 
nN 
