SPERMATOPHYTES—ANGIOSPERMS 
307 
The peculiar embryo sac of Plumbagella, with only two nuclei, 
the egg nucleus and the endosperm nucleus, when ready for fertili¬ 
zation, is shown in Figure 112. 
Fertilization.—’The later stages cut to show the mature embryo 
sac will often show fertilization. The male and female nuclei almost 
invariably show a difference in staining capacity when the male 
nuclei are just discharged 
f rom the pollen tube. With 
cyanin and erythrosin, the 
male nucleus stains blue and 
the female red; hence the 
obsolete terms cyanophilous 
and erythrophilous. As the 
nuclei come into contact 
within the egg, they begin 
to stain alike, the male 
nucleus staining more and 
more like the female. In 
the final stages of fusion it 
is difficult, or impossible, to 
distinguish the two nuclei. 
The male nucleus which 
takes part in the “triple 
fusion” to form the endo¬ 
sperm nucleus behaves in 
the same way. 
Lilium is a very good 
and always available type 
for illustrating fertilization 
(Fig. 113). Take ovaries 
from flowers whose petals have withered but have not yet fallen off. 
Though their embryo sacs and nuclei are smaller, Silphium and Heli- 
anthus are good types, because their curved or twisted male nuclei 
are easily distinguished from the spherical nuclei in the embryo sac. 
The embryo sacs of orchids are very small, but ovules are extremely 
numerous and the chances for securing the fusion of nuclei are cor¬ 
respondingly good. In Cypripedium the nuclei do not fuse in the 
resting condition, but the chromosomes of the two parents are per¬ 
fectly distinct in the egg. The general statement that nuclei fuse 
Fig. 111.— A, head of Aster; B, pod of Capsella; 
C, transverse section of ovary of Lilium. The dotted 
lines show how the material should be trimmed before 
fixing. 
