468 
F. L. PICKETT 
It now becomes even more highly vacuolate, the spaces about the 
spores disappear and an actual contact between the protoplasm and 
the spore walls is evident in places. 
Up to the time of the general migration through the sporangial 
cavity, the tapetal nuclei largely retain their original form. With 
the movement of the protoplasm they slip between the tetrads and 
at the same time become slightly larger and considerably distorted in 
form. A marked change in nuclear structure appears. Each nucleus 
shows many large, irregular vacuoles and often two or more nucleoli 
each surrounded by a large, definite vacuole (text figure 5). They 
continue to increase slightly in size and show more widely varying 
forms, up to the time of exine formation by the pollen spores. It has 
been as yet impossible to demonstrate active movement on the part of 
these nuclei; but it is difficult to find any other explanation of the 
appearance of their peculiar forms so long after their first migration 
through the cavity. There has been found no indication of mitosis 
in these nuclei, and only occasional forms which seem to indicate 
increase in number by simple division (text figure 6, A) have been 
observed. 
With the maturing of the pollen grains the protoplasm surrounding 
them becomes less dense, with finer strands separating the more 
sharply marked vacuoles (text figure 6), and finally disappears. 
Along with this change in the protoplasm the wandering nuclei shrink, 
show a decrease in general density and a gathering of the chromatin 
into small, almost opaque masses (text figure 6). In many cases the 
nuclei become more regular in form before finally shrivelling up with 
the loss of water when the pollen grains mature. In a few cases they 
retain much of their characteristic appearance until quite late (text 
figure 8), although most of them have lost all indications of activity 
before the stage of maturity indicated in this figure. 
Summary 
The study reported at this time shows that in the development 
of the microsporangia of Arisaema triphyllum (L.) Schott. and A. 
Dracontium (L.) Schott. the tapetum is early differentiated by peculiar- 
ities of cell wall, nuclear and cytoplasmic structure. 
The walls of the tapetal cells entirely disappear, allowing the 
several protoplasmic masses to form a periplasm spreading through 
the sporangial cavity. 
