SPERMATOPHYTES—ANGIOSPERMS 
305 
and for the achromatic structures, the safranin, gentian-violet, 
orange combination has not been excelled. The photomicrographs 
(Figs. 108-110) illustrating the series from the archesporial cell 
(which, in this case, is also the primary sporogenous cell and the 
megaspore mother-cell) to the four megaspore nuclei will repay a 
careful study. One more mitosis produces the 8-nucleate embryo sac, 
but Lilium is not a good type for illustrative purposes, since the egg 
apparatus is not very definitely organized. 
Fig. 109 .—Lilium philadelphicum: photomicrograph of transverse section of ovary showing, 
in one of the ovules on the left, the first mitosis in the megaspore mother-cell; and, in one of the 
ovules on the right, the second mitosis which gives rise to the four megaspore nuclei—chromo- 
acetic acid; safranin, gentian-violet, orange. Cramer contrast plate; 16 mm.-objective; ocular 
X4; yellowish-green filter and also a strong filter such as is used in outdoor work; camera bellows, 
30 cm.; exposure, 2 minutes. Negative by Miss Ethel Thomas. X64. 
For the embryo sac at the fertilization stage, many of the 
Compositae are good. Senecio aureus is quite favorable, because it is 
easy to cut and the akenes do not spread. Aster gives an exceptional 
view of the antipodal region, but is rather hard to cut. Before 
fixing, trim the head as indicated in Figure 111. Silphium, espe¬ 
cially S. laciniatum, furnishes an ideal view of the embryo sac. 
With thumbs and fingers grasp the two wings of the akene and care¬ 
fully split it, exposing the single white ovule inside. This is rather 
