390 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
latter (1884) saw the same bodily exit of both male nuclei from the 
open pollen tube of Picea, but found only one male nucleus fusing 
with that of the egg. In the same publication Strasburger also 
records numerous instances in which he had been able to observe the 
same mode of escape of the contents of the pollen tube into the ripe 
embryo sac in angiosperms. At last, as Strasburger puts it, in dis- 
cussing fertilization in the conifers: 
The most important morphological facts are clear. It is established that the male 
nucleus that copulates with the egg nucleus, passes as such out of the pollen tube into 
the egg. 
Thus, finally, was the actual material contribution of both parents 
to the embryo of the seed plants first seen. This was just two cen- 
turies, lacking a decade, after Camerarius (1694) had proven that the 
presence of pollen on the stigma is indispensable to seed formation. 
One chief reason why this important problem so long baffled all in- 
vestigators was the lack of proper methods of preparing material for 
study. The older method of studying unfixed and unstained sections 
had certain advantages, it is true. The sequence of developmental 
stages was often determined with certainty by actually following 
their succession in living material under the microscope, and there 
was less cause also for dispute about artifacts. But structures of 
the same refractive qualities were not readily distinguished in such 
sections. As Strasburger himself says (1884, p. 18): 
The negative results of my earlier studies and of those of Elfving were due to the 
lack of a method which permitted the nuclei to be distinguished in the strongly re- 
fractive contents of the pollen tube up to the moment of fertilization. 
That these studies of 1884 were successful was largely due to the 
use of material fixed in five-tenths per cent acetic acid, 1 per cent 
osmic acid or in absolute alcohol, and stained in borax carmine, hema- 
toxylin or iodine green. 
The extreme significance of the fact that those most highly organ- 
ized portions of the cell substance—the nuclei—were so prominent 
in the process of fertilization was at once appreciated by Strasburger, 
who in 1884 (p. 77) announced the following general conclusions as 
the outcome of his consideration of the phenomena observed: 
(1) The fertilization process depends upon the copulation with the egg nucleus of 
the male nucleus that is brought into the egg, which is in accord with the view clearly 
expressed by O. Hertwig. (2) The cytoplasm is not concerned in the process of ferti- 
lization. (3) The sperm nucleus like the egg nucleus is a true cell nucleus. 
In the years since 1884 the nuclei have been found to be the struc- 
tures chiefly concerned in fertilization, whenever such a process occurs. 
Among the earlier observations of this nuclear union at fertilization 
in each of the great groups are the following, named in the order of 
discovery: It was seen in Pilularia (Campbell, 1888), in Riella (Kruch, 
