98 COMPARATIVE EMBRYOLOGY OF THE RUBIACEAE 
The course of the pollen tube through the style needs no re- 
mark since what has been said above applies throughout. One 
point, however, needs mention, namely, that here the cellulose 
plugs described by Treub (22) as occurring in the pollen tubes in 
Casuarina, by Nawaschin (12, 16) in the Amentiferae and by 
Murbeck (10) in Alchemilla are here very easily and clearly seen- 
Two of these plugs are shown in figs. ro and rr. It will be seen 
from the outline of the plug and the form of the somewhat 
. shrunken pollen tube contents that these fit each other. One finds 
sometimes that the tube on one side of the plug is empty ( £g. 70). 
In other cases cytoplasm occurs in both sides (fig. rr). Similar 
plugs occur lower down in the fusion tissue, but they have not 
been observed elsewhere. 
: When the pollen tube reaches 
the epidermis of the basal partition 
(text fig. ro), it stops at that point, 
and does not enter between the 
cells as occurs in 0۰ 
They then bend upward and travel 
between the two tissue masses until 
they come to the collar of con- 
ductive epidermis of the strophiole, 
in the mucilaginous outer wall of 
: which they pursue their way. Af- 
FiG. 10. Two views of the ovule ter moving upward, 2 e, toward 
of Diodia Virginiana, showing the the chalaza, a little distance, they 
conductive epidermis (dotted) and the turn, following the band of se- 
path of the pollen tube; و‎ funicle. . & : } : 
( Schematic. ) creting epidermis as shown in 
text fig. 10, and being com- 
pletely buried in the mucilage, as occurs in plants studied by 
Dalmer (3). The behavior of the pollen tube toward the walls of 
the secreting cells needs particular mention in view of the fact that 
the wall of the former coalesces with the outer walls of the latter 
in such arway as to be completely indistinguishable. In one pre- 
paration the pollen tube was cut transversely at one point of its 
course and showed this relation especially well. In fig. 9 this is 
well shown, from which it will be seen that the outer part of the 
tube wall is thin, thickening markedly on the sides nearer the epi- 
