218 Nichols. A morphological study of Juniperus communis var. depressa. 
in fig. 1. The integument is well defined, and the nucellus is 
composed of a mass of cells among which a slight differentiation 
is already apparent. Those near the tip have ceased to divide 
and have become vacuolate. Before long their growth stops entirely 
and their walls become slightly thickened. In the basal portion 
of the nucellus, in marked contrast to the apical region, the cells 
continue to divide by periclinal walls, giving rise to longitudinal 
rows of prismatic cells (hg. 59). At the lower end of these rows 
several cells soon become prominent by reason of their large size, 
big nnclei, and dense cytoplasm. This group constitutes the 
archesporium, which it will thns be seen is organized over twelve 
months before the formation of the prothallium. Düring the summer 
the archesporium increases somewhat in size, yet at no. time is 
there a sharp line of demarcation between the sporogenous cells 
and the other cells of the nucellus. Certain cells of the arche¬ 
sporium take the lead in growth, but it is impossible to state with 
Longitudinal section through young ovule immediately before pollination. 
Micropyle open. X 66* 
certainty which of these will become the spore mother cell. The 
vegetative cells surrounding the archesporium, as noted by Noren 
(1907), become somewhat flattened and form more or less concentric 
layers. It is of interest to note that in Cephalotaxus (Coker 1907), 
where pollination likewise takes place the year preceding fertiliza- 
tion, the development of the female gametophyte proceeds at the 
same rate as in J communis , while in Pinus (Ferguson 1904), 
where similar conditions are found, the embryo sac Starts to devel- 
op the first season and passes the winter in the thirty-two nucleate 
stage. 
Tetrad division. — The writer has made no attempt to 
obtain a complete series of the various stages in spore formation, 
but they are doubtless similar to those described by Noren (1907). 
The megaspore mother cell can be positively identified for the first 
time early in April, when its nucleus enters synapsis (fig. 60). Dür¬ 
ing this period the nucleus presents an appearance very similar to 
that described in the nuclei of the microspore mother cells, the 
