SEXUALITY IN PLANTS—JOHNSON. 387 
In 1846, however, the error of this conception was clearly demon- 
strated by Amici, who showed that the embryo of the orchids arises 
from an egg which is already present in the embryo sac when the 
pollen tube reaches it. It is this preexisting egg, according to Amici, 
that is stimulated to form the embryo by the presence near it of the 
pollen tube. This view was confidently supported by Mohl (1847) 
and Hofmeister (1847), and the controversy with Schleiden became 
even more spirited. As Mohl afterwards wrote (1863), men were 
“led astray by their previous conceptions to believe they saw what 
they could not have seen.”” The dispute even approached the ac- 
rimonious, as when Schleiden (1843) says of one worker’s figures, 
“Solche Praparate sind ohne Zweifel aus den Kopf gezeichnet.” 
Hofmeister, from the beginning of his study of fertilization in 
seed plants, had sought in the pollen tube for some equivalent of 
the spermatozoids, those motile male cells of the mosses and ferns that 
had first been understood by Unger in 1837. He was unable, how- 
ever, to do more than point out the mistake of earlier observers in 
regarding the starch grains of the pollen tube as spermatozoids, and 
to suggest the likelihood that these motile cells might be discovered 
in the gymnosperms, a prediction the fulfillment of which was realized 
by Ikeno and Webber 50 years later. In his study of pollen tubes 
Hofmeister demonstrated to his own satisfaction that the tube does 
not open in accomplishing fertilization. His view, which was the one 
current till 1884, was that the egg is stimulated to develop into the 
embryo by some substance that diffuses through the imperforate 
wall of the pollen tube. 
IlIl.—_THE DISCOVERY OF A PROTOPLASMIC FUSION AT FERTILIZATION. 
We come now to consider a series of discoveries of supreme im- 
portance in the investigation of the essential sexual process in plants. 
This is the period in which the problem that had baffled naturalists 
for twenty centuries was at last solved, at least in one most essential 
feature, by the demonstration of the occurrence at fertilization of a 
mingling of paternal and maternal substances. 
It will not be without interest at this point to note the intellectual 
stimuli which led an unusual number of workers to investigate this 
phase of our problem. 
In the first place, there were on record and under discussion at the 
middle of last century the many puzzling observations of the “spiral 
faden,”’ or animalcule, as they were thought to be, that had been 
found arising from a number of plants. These motile, spiral fila- 
ments had been seen in a liverwort (Fossombromia) by Schmiedel 
(1747), in Sphagnum by Esenbeck (1822), in Chara by Bischoff 
