924 KirKwoop: POLLEN-FORMATION IN CUCURBITACEAE 
divide by periclinal walls. These cells are distributed along the 
ridges which later plainly mark off the positions of the locules. 
The series thus formed gives rise to three layers of cells, one of 
which forms the tapetum. Outside of the tapetum only three 
layers of cells including the epidermis are to be found in the ma- 
ture anther. 
The primary sporogenous cells enter directly upon a period of 
growth, becoming spore-mother-cells without divisions. The 
mother-cells form a linear series, sometimes broadening into a 
narrow plate but never dividing to form a cylindrical mass as in 
the case of Fevillea. These cells become large and prominent 
and each gives rise to four microspores. Here as in the preced- 
ing case the nucleus of the microspore does not divide to form 
vegetative and generative nucleus until quite late. 
In Cyclanthera when the rudiment of the androecium has be- 
gun to assume the form of a disk the hypodermal cells prepare 
for periclinal divisions in two zones, one above the other. The 
cells in the two narrow bands thus situated divide as in the pre- 
vious types to form the primary parietal and primary sporoge- 
nous cells (FIGURE 11). The former divide again to three layers, 
which with the epidermis constitute the outer wall of the locule 
(FIGURE 12). The primary sporogenous cells do not divide to in- 
crease the number of spore-mother-cells, but merely enter upon a 
period of growth until the time arrives for the formation of the 
macrospores. Four microspores are formed from each spore- 
mother-cell in the usual order. They remain for some time ina 
pyramidal arrangement surrounded by a transparent, seemingly 
gelatinous envelope, and during this time the exine begins to 
thicken and the nucleus assumes a less chromatic aspect. AS 
they separate the microspores become rounded off and begin to 
develop their characteristic markings. Almost mature  pollen- 
grains were observed in the sections, but each contained only one 
nucleus. The tapetum retains its active appearance until the 
spores are almost mature. 
But the chief interest in pollen-formation in these forms is not 
in the facts just cited, but in the details of the process as they ap- 
pear in the division of the mother-cells. In the few members of 
the Cucurbitaceae here under consideration, the course of develop- 
