62 Rhodora [MancH 
large enough to fill the sac. On all sides the tissues are then 
pushed back. Below — or towards the exostome of the seed — the 
end of the embryo, where the suspensor should be found, is rounded 
and is covered only by the remains of the enveloping cells (Fig. 2). 
At the opposite extremity a recession of the sac is 
filled with dense protoplasm in which a large and 
perfect nucleus (ez) is always present. 
The question of the origin of this nucleus is of 
great interest just at the present time. For those 
who have not freshly in mind the history of the 
various nuclei of the embryo-sac of angiosperms, 
it should be said that at a certain stage four nuclei 
are found at either end of the sac. One nucleus 
from each of these groups then advances to the 
Fig. 2 centre of the sac. These are termed polar nuclei. 
They ultimately fuse and the resulting body is the so-called endo- 
sperm nucleus, ordinarily to be distinguished by its size and position. 
The three nuclei remaining at the base of the sac are the antipodals. 
One of the three at the micropilar end becomes the egg-nucleus; the 
other two are the nuclei of the synergides. After the inception of the 
embryo, in most plants the endosperm nucleus divides repeatedly, 
giving rise to the cells of the endosperm the function of which is to 
nourish the embryo. 
Recently Nawaschin, followed by other observers, has discovered 
that in some plants at least, one of the male nuclei from the pollen 
tube takes part with the polars in the formation of the endosperm 
nucleus. Both male generative cells are thus functional, one com- 
bining with the egg, one with the polars. Nawaschin calls this 
double fertilization. The discovery is regarded as one of the most 
interesting of recent years. 
In following the fertilization of some tropical orchids Nawaschin 
has come to the conclusion that there is no fusion of polars and 
male; that though the three come to lie in a close group, no actual 
union occurs. This appears to Nawaschin to explain the absence of 
endosperm in Orchidaceae. 
Strasburger long ago described the nuclear changes in the 
embryo-sac of several native orchids, and specifically affirmed the 
fusion of the polars. Now he has reéxamined his material and 
concludes that the extra spermatic nucleus takes part with the 
