KirRKWOOD: POLLEN-FORMATION IN CUCURBITACEAE 225 
ment leading up to the formation of the pollen differs in no impor- 
tant particular. Some apparently minor differences have been 
noted which will be mentioned. The history of the microspores 
has been most carefully followed in Mécrampelis, which is a favor- 
able object for such a study. The principal stages in the forma- 
tion of the microspore have also been noted in Fevillea, Melothria, 
Cucurbita moschata, C. Pepo, and Cyclanthera. 
So far as the writer is aware no examination has been made 
of the cytology of the pollen-mother-cells of any of the Cucur- 
bitaceae, though certain features of the vegetative cells have been 
studied by Zacharias.” 
Though less favorable in some respects than some other subjects 
for such studies, certain features have been observed which seem 
worthy of attention. 
The history of the microspore from this point of view begins 
with the formation of the primary sporogenous cell. At the con- 
clusion of the mitosis which forms it, the sporogenous cell is not 
distinctly different in appearance from any of its hypodermal 
neighbors. A considerable number of those in its immediate vicinity 
have a decidedly meristematic character. At this time the cyto- 
plasm of these cells is relatively thin as compared with the later 
states. The mitotic figure which is concerned in the first division 
of the hypodermal cell is a multipolar one, at least in the meta- 
phase, and its chromatin is massed together in a compact zone at 
the equator. 
The primary sporogenous cells become directly the pollen- 
mother-cells. They enter upon the growth period immediately after 
their formation, and soon become very conspicuous by reason of 
their size and structure. The expansion of their nuclei is the first 
distinctive feature shown in their growth, and this without any 
apparent increase in chromatic substance makes the nuclei appear 
quite clear, except for the single large nucleolus and their chro- 
Matin network, In appearance the cytoplasm consists of a very 
fine network of granular threads. The granules stain darkly and 
the cytoplasmic body has a grayish cast with the haematoxylon 
stain. At this time there is no perceptible arrangement of the 
Cytoplasmic substance and it is equally dense in all parts. With 
the growth of the cell the granules increase in size slightly up to 
