24 Dixon . — Fertilization of 
In most cases the sister-cells arising from this division are 
almost of the same size as in Biota , Juniperus , Ginkgo , and 
the Abietineae ; whereas in Taxus baccata they are very 
unequal in size. It is usual in Biota and Juniperus for these 
two cells to lie side by side in the end of the pollen-tube, but 
in Taxus the larger one is usually below the smaller and closer 
to the end of the tube. When the nucleus of the stalk-cell 
has passed by the body-cell or its two daughter-cells and taken 
up its position near the nucleus of the pollen-tube, it becomes 
extremely similar to this nucleus. This similarity is probably 
due to the fact that the two nuclei are nourished by the same 
cytoplasm. 
The history of the nuclei of the pollen-grains of the Abie- 
tineae had not been yet so exactly traced as in other groups, 
and accordingly it was hoped that some points still unobserved, 
and with regard to which surmises only had been formed, 
might be made out by investigation in this group. For the 
investigation Prof. Strasburger was kind enough to supply me 
with pollinated cones of Pinus silvestris taken at intervals of 
a week, dating from April 24 to June 6, 1893. The cones, of 
course, were pollinated in 1892, as the pollen-grain of Pinus 
silvestris reaches the nucellus about thirteen months before 
fertilization takes place. 
Prof. Strasburger had already determined that, in the ripe 
pollen-grain of Pinus silvestris , there are a small prothallium- 
cell, the last of three formed, and a large nucleus. This 
latter is the nucleus of the pollen-tube and passes into the 
tube which is formed immediately after pollination. The 
prothallium-cell remains attached to the wall of the grain, 
opposite the place of exit of the pollen-tube. In this 
condition the pollen remains during the winter. 
The following research is concerned with the subsequent 
history of the pollen. 
The next spring the pollen-tube becomes filled with starch, 
and, towards the end of April, the prothallium-cell divides to 
form a small stalk-cell and a larger body-cell. PI. Ill, Fig. 1 
shows this division just completed. Very shortly after this 
