18 DUNCAN, ON THE IMPREGNATION 
is not dependent upon endosmosis and exosmosis; it is a 
growth of the cell-wall of that layer of the pollen-grain which 
contains the granules and fluid usually termed protoplasma. 
The growth is peculiar to the perfect pollen-grain, and occurs 
at a certain period when the viscid secretion of the papille of 
the stigma is strong enough to hold the pollen-grain in perfect 
apposition, and to resist the effects of the pressure exercised 
by the end of the pollen-tube upon the tissue of the stigma 
before entering. Were this viscidity insufficient, the gentle 
force of perforation could not take place; and when the 
viscidity is sufficient the growing tube, with its conical tip, is 
held forcibly against the cell-structures of the stigma; the force 
to cause these to diverge and to admit the tube between them 
is “ growth.” “The amount of force employed may be roughly 
estimated by adding water to the viscid secretion, some hours 
after perforation has taken place ; the pollen-grain is released 
from its durance vile and jumps away from the stigma; its 
restraining fluid having been rendered inefficient. 
Once entered between the cells of the stigma, the pollen- 
tube, consisting of a cell-wall enclosing the spermatic mate- 
rials, closed by a conical end, and continuous with the pollen- 
grain, begins to elongate with extraordinary rapidity (fig. 5, 
a,b). The following are the results of experiments by Dr. 
Maclean and myself : 
1. Four hours after the application of pollen to the stigma 
the pollen-tubes were detected one inch down the style; day 
fine, and good sun. 
2. Highteen hours after application, the pollen tubes were 
detected at the base of the style, three and a half inches from 
the end of the stigma. 
3. Twenty-four hours after application, the pollen-tubes 
were seen in the micropyles of several ovules. 
4. Thirty hours, impregnation is complete, and the pollen- 
tubes are wasting in the micropyle. 
Series 1].—Ezperiments by Dr. Maclean and myself. 
lst. Tigridia fertilised with pollen-grains by twelve o’clock 
in the day (not much sun). 
The perianth closed as usual about five o’clock, and at 
nine o’clock two inches of the style were removed with the 
stigma, and the rest of the flower placed in water. 
In these two inches of style, hundreds of pollen-tubes ex- 
isted, and the diameter of the style was considerably increased 
by their presence. They were cellular, and the cells of the 
