92 COMPARATIVE EMBRYOLOGY OF THE RUBIACEAE 
THE COURSE OF THE POLLEN TUBE 
In the free upper portion of the style the cells of the conduc- 
tive tissue are, as above pointed out, long and very narrow. 
Through this tissue the pollen tube takes a somewhat devious, 
though on the whole direct course ( fig. 2), and as the tube has a 
. transverse diameter much greater than that of the neighboring 
conductive tissue cells, one would expect that the mechanical 
pressure alone would cause not a little disturbance in the conduc- 
tive tissue. Such, however is not the case. The pollen tube does 
not destroy the cells among which it passes either by chemical in- 
fluences, such as digestion, or mechanically. It is true that the 
cells are frequently disturbed in position, as is plentifully evidenced 
by the form of the nuclei when the pollen tube happens to exert 
pressure on a cell in such a way as to affect its nucleus (fig. 3). 
Without exception, however, the nuclei and cytoplasm stain per- 
fectly normally and evenly. The walls of the pollen tubes are to 
a considerable degrees thicker than those of conducting cells, 
while the cytoplasm of the former is more coarsely granular than 
that of the latter. | 
When the pollen tube reaches the stylar partition it meets with 
conductive cells with thicker walls and of larger transverse dimen- ' 
sions. The nuclei are here long-oval in form. . Here the path has 
about the same character as in the free part of the style. Occa- 
sionally one finds a sharp turn such as is shown in fg. 3, which 
represents a small part of the conductive tissue in longitudinal 
section showing a pollen tube cut transversely. A study of this 
preparation shows clearly that the effect of the pollen tube upon 
the tissue in this part of its course through which it passes is 
mechanical. Instances are frequently found in which the pollen 
tube passes athwart the conductive tissue, either directly or more 
or less obliquely. At first glance it is somewhat difficult to at- 
tribute such behavior on the part of the pollen tube to mechan- 
ical influences. When, however, we reflect upon the extreme 
sensitiveness of the protoplasm of the pollen tube to stimuli, 
it becomes less difficult. The impingement of the end of the pol- 
len tube upon the end wall of a cell is, we believe, enough to turn 
it out of its direct path. Upon approaching the point of fusion of 
the basal and roof elements of the ovary septum the pollen tube 
