152 
WALTER P. THOMPSON 
it consists of at least two cells and usually of three or four (fig. 19). 
It makes its appearance at about the time that the inner envelope 
(ovary) becomes differentiated. The succeeding events take place 
in the usual manner. The archesporial cells by periclinal walls cut 
off the primary wall cells (fig. 20) which, together with the epidermis 
develop a large mass of tissue above the sporogenous cells. The cells 
of the upper part of the nucellus become densely charged with cyto- 
plasm which contains many starch grains. No definite pollen chamber 
is formed though the tip of the nucellus usually becomes disorganized 
by the ingrowing pollen tubes. That part of the nucellus at the base 
of the embryo-sac develops a peculiar nutritive pavement tissue which 
according to Coulter (7) was mistaken by Lotsy (19) for tissue within 
the sac. But it will appear later that Lotsy was correct in stating 
that there may be cellular tissue in the base of the sac when there 
are only free nuclei at the top. It will also appear, however, that 
Lotsy was mistaken in stating that this cellular tissue was present 
before the pollen tube enters the sac. It is in reality endosperm tissue 
which develops after the entrance of the pollen tube and is quite 
distinct from the pavement tissue outside the sac. 
The whole ovule undergoes a rapid development while the pol- 
len tubes are growing through the nucellus. Soon after pollination 
time (indicated by the flaring style with its droplet of liquid) certain 
ovules are seen to be growing rapidly while others remain the same 
size. One naturally concludes that the growing ovules have been 
fertilized and that the endosperm is forming. On sectioning such 
ovules, however, one invariably finds that the pollen tubes have not 
yet reached the sac. Not until the ovules are considerably larger 
than those which failed to be pollinated does one find pollen tubes 
in contact with the embryo-sac. It seems, therefore, that the presence 
of pollen tubes in the nucellus stimulates the latter to further develop- 
ment. 
(c) Sporogenous Tissue. — According to Strasburger (25) the 
primary sporogenous cells divide to form the mother cells, a very 
unusual behavior in Gymnosperms. My sections show clearly that 
this is not the case but that the primary sporogenous cells as usual 
function directly as mother cells. There are accordingly from two 
to four of the latter. Figures 20 and 21 show these cells in longi- 
tudinal section and figure 22 in transverse section. The mother-cells 
divide to form the megaspores while the ovule is still very young — in 
