1912] STEVENS—HETEROSTYLOUS PLANTS 297 
empty mother cell wall with its four chambers often persists for 
some time (fig. 22). 
TAPETAL CELLS 
About the time the pollen mother cells reach the pachyténe 
stage, the tapetal cells begin a rather irregular free nuclear division. 
The nuclei show all gradations from true mitosis to what is appar- 
ently simple amitotic division. Figs. 23-25 show the amitotic 
division of these nuclei. By the time the pollen mother cells have 
finished the homotypic division, the tapetal cells ey contain 
two and sometimes four free nuclei (fig. 26). 
MEGASPORE MOTHER CELLS 
The difficulty of orienting the buds makes the number of prepa- 
rations necessary to secure a full series of stages so great that a 
complete study of the development of the megaspores has not yet 
been made. One preparation of the long-styled form, however, 
showed typical diakinesis, which, as was to be expected, had much 
the same appearance as that in the microspore (fig. 27). Usually 
only one megaspore mother cell is formed in an ovule, and each 
flower produces but a single seed. One ovule, however, which 
happened to be in the long-styled form, contained two apparently 
well-developed and normal megaspore mother cells, with their 
nuclei in the pachyténe stage. Their shape seems to indicate that 
they were formed by the division of a single cell by an anticlinal 
wall (fig. 28). 
Houstonia caerulea 
The Rubiaceae contain nearly half the genera known to be 
heterostylous. The flowers of Houstonia are plainly dimorphic, 
the pistil being exserted in one form and the stamens in the other. 
The pollen grains vary somewhat in size in each form, but those of 
the short-styled form are larger than those of the long-styled, their 
diameters being in about the ratio 10:7. No differences have been 
noted in the vegetative structures of the two forms. The flowers 
are so small that experimenting with them would be very difficult, 
and accordingly very little is known about the relative fertility of 
legitimate and illegitimate unions. Darwin, however (11, p. 132), 
