DEVELOPMENT OF THE POLLEN TUBE AND PROTHALLUS. B37 
Hirase (61) has also found that there is a similar return of the tube 
nucleus to the pollen-grain end of the tube in Ginkgo, so that the 
process would seem to be a general one in cases where this type of 
fecundation occurs. | 
During the downward extension of the basal end of the pollen tube 
the prothallial apparatus remains attached to the base of the tube and 
‘is carried down with the tube in its elongation. The central cell in 
the course of this development is frequently compressed and drawn 
out so that its shape is greatly altered (figs. 48,49, and 51). In stages 
preceding this development the central cell is normally elliptical or 
oblong, its major axis corresponding to the major axis of the pollen 
tube (fig. 20). After the tube has turned downward and is sufficiently 
developed so that the basal end is free from surrounding tissue, the 
central cell rounds up and becomes nearly spherical. 
In the course of this development and change of the basal end of the 
pollen tube and the prothallus, the blepharoplasts have also changed 
their position. Previous to this development they occupied the poles 
of the nucleus, a line passing through them corresponding to the 
major axis of the cell and longitudinal axis of the pollen tube. During 
the growth of the basal end of the pollen tube and consequent change 
of the prothallus they have changed their position in regard to the 
pollen tube and nucleus and have come to lie at opposite points on the 
equator of the nucleus transverse in the pollen tube (as shown in figure 
21, though this is after the division of the cell). The writer has not 
been able to determine whether this change in position of the blepharo- 
plast is due to a definite motion of the blepharoplasts themselves or to 
a change of shape of the cell and nucleus. It would seem, however, 
that the blepharoplasts must move in the cell independent of the 
motion of any other organ. As the central cell is pulled down by the 
growth of the pollen tube it is not infrequent to find the two blepharo- 
plasts preceding the nucleus. However, in the course of this trans- 
formation the blepharoplasts may be found in almost any position in 
the central cell, but usually remain nearly on opposite sides of the 
nucleus. 
The entire prothallial apparatus continues to increase in size until 
the latter part of May in Z. foridana and about June 10 in Z. pumila, 
when the central cell and blepharoplasts have reached their full size 
and the division of the central cell begins. In this stage of develop- 
ment all cells of the prothallus retain their relative positions with 
reference to each other, but the base of the pollen tube in which the 
prothallus is located has in most instances begun to grow and change 
its position as described above, and a change in the shape of the cen- 
tral cell has frequently resulted before this stage is reached. The first 
prothallial cell and stalk cell in this stage retain the same position as 
described in the preceding stage, but have greatly increased in size 
(ug. 21). 
