DEVELOPMENT OF THE POLLEN TUBE AND PROTHALLUS. 23 
of the exine of the grain at a point opposite the attachment of the 
prothallus. No matter in what direction the pollen grain may lie, 
the tube as soon as protruded grows toward the tissue of the nucellus, 
forming the side of the pollen chamber, into which it soon penetrates 
(fig. 5). 
If the pollen grain is so situated that the tube when first protruded 
points toward the apex or base of the nucellus, it makes a sharp turn 
immediately after leaving the pollen grain and enters the nucellar 
tissue. In Zama the tube never branches before entering the tissue, 
and while it occasionally branches after entering the tissue, this is 
by no means of common occurrence. The majority of tubes remain 
unbranched throughout their growth. In Gingko, on the contrary, 
the distal end of the tube as soon as it enters the nucellar tissue 
becomes very much branched and the ramifications are so slender that 
it is only with the greatest difficulty that they can be traced. 
When the pollen tube first ruptures the exine and protrudes, it is 
considerably smaller than the diameter of the pollen grain. The rela- 
tion of the prothallial cells and tube cell which forms the tube is 
shown in this stage in figure 12. As the tube pushes out, the proto- 
plasm of the tube cell draws away from the wall of the old pollen 
grain to some extent apparently in all instances, though it would seem 
probable that the contraction shown in the figure is somewhat abnor- 
mal. ‘The nucleus of the tube cell, which is densely granular, imme- 
diately passes into the tube, becoming the pollen tube nucleus, and 
travels farther as the tube grows, remaining always about a uniform 
distance from the apex of the tube. In this early stage of germina- 
tion the cells of the prothallus have the same size as in the mature 
pollen grain and appear about the same. 
The entire prothallus in this stage is about 9 “ wide and 8 u“ long 
(length being considered the extension in the direction of the growth 
of the pollen tube). The nuclei in both the first and second prothallial 
cells are still very densely granular and almost fill up the entire cell 
in each case. The nucleiin both cells are more or less crescent-shaped 
in median section, corresponding to the shape of the cells. In the 
tube shown in figure 10 the nucleus of the first prothallial cell meas- 
ured 7.12 by 3 mu; that of the second prothallial cell 8 by 3 4, and the 
nucleus of the tube cell 8.01 by 5.34 4, the nucleus of the tube cell in 
this stage always being slightly larger than the nuclei of either of the 
prothallial cells. 
When the Flemming triple stain was used in the study of pollen 
tubes the safranin always stained the wall of the pollen grain red, 
serving as an important distinction for a pollen grain wall throughout 
the development of the apparatus. 
Before the pollen tube has increased in length very greatly it 
increases also noticeably in width, and by the time it has reached a 
length four or five times as great as the diameter of the pollen grain 
