Pinus silvestris . 
25 
it is found that the body-cell has broken free from the stalk- 
cell and has* divided into two cells, which are almost equal 
in size (Figs. 2, 3). These cells are the male sexual cells. 
During this process the wall of the stalk-cell is ruptured and 
its nucleus follows the two cells resulting from the division 
of the body-cell which move into the pollen-tube (Fig. 4). 
The wall of the stalk-cell is ruptured in such a manner that 
there remains a ring-shaped portion of it standing up from 
the inner wall of the pollen-grain. This persists for a long 
time and may still be seen in much later stages, as in Fig. 10. 
The two male cells move slowly down into the pollen-tube 
and are closely followed by the nucleus of the stalk-cell 
(Fig. 5), which ultimately overtakes them. Fig. 6 shows the 
nucleus of the stalk-cell just passing the two sexual cells. 
In the section from which Fig. 7 is drawn, the nucleus of the 
stalk-cell is well past the sexual cells and all three are 
wandering down the pollen-tube. The same relative position 
is shown in Fig 8. As these three, the two sexual cells 
and the naked nucleus of the stalk-cell, pass down the pollen- 
tube and finally approach its lower extremity, they naturally 
come into a position close to the nucleus of the pollen-tube. 
This latter nucleus at this stage is usually very different 
in appearance from the nucleus of the stalk-cell ; in fact, 
it resembles the nuclei of the sexual cells, its nucleolus 
appearing like a very refractive ring, while the nucleus of the 
stalk-cell is coarsely granular (Fig. 9). I did not find the 
sexual nuclei so close to the lower end of the pollen-tube 
as is shown in Fig. 9 till May 12. As soon as the nucleus 
of the stalk-cell leaves the pollen-grain and passes into the 
pollen-tube, the former is usually more or less completely 
emptied of its contents. In this condition it is often very 
easy to see the ring-shaped remnant of the wall of the stalk- 
cell, Fig. 10. Fig. ii shows the remains of two prothallium- 
cells as well as the ring-like wall of the stalk-cell. 
During the period between April 26 to May 1 2, the growth of 
the pollen-tubes was found to be extremely slow, their passage 
being made through the hard tissue at the top of the nucellus, 
