20 DUNCAN, ON THE IMPREGNATION. 
the conical end of the pollen-tube at the rate of an inch in 
six hours, at the rate of an inch in four hours, and sometimes 
at even double that rate, through cellular tissue whose forma- 
tion is very much adapted for the transition. It is demon- 
strable, from repeated experiments, that this force is exercised 
most efficiently when the direct sunlight and heat are accom- 
panied by a warm and humid atmosphere, and most ineffi- 
ciently when there is no sun. In fact, the greatest stimuli 
to vegetable growth are those which strengthen all the powers 
of the pollen-tube. 
From the above experiments it is to be proved that the 
force just spoken of is exercised when the pollen-grain, and 
even one half of the pollen-tube, are removed. 
It is manifestly no force arising from the pollen-grain as a 
fixed point. The whole secret is contained in the pollen- 
tube itself; and in Tigridia, if by careful manipulation in 
making longitudinal sections of the style and tearing with 
the needle a few tolerably lengthy pollen-tubes are exposed, 
it will be noticed that the pollen-tube is not one continuous 
elongation of the cell-wall of the pollen-grain, but that it is 
CELLULAR (fig. 5, a, 4). Transverse inflections of the 
tubular cell-wall exist every now and then, and the pollen- 
tube is really a tube formed by elongated cells. These cells 
resemble, in a most singular manner, those of the Conjuga- 
teze, when their spiral contents are removed ; the cell-wall is 
beautifully definable by the highest powers, and it is evident 
that the cylindrical shape of the tube is often lost when, pass- 
ing between the long cells of the style, no great space can be 
obtained. I have found the cells of the pollen-tube in all 
parts of the style, and also within the canal of the micropyle. 
The force of the progression of the pollen-tube is then cell 
growth; the cells, in their passage through the style and 
axis, are nourished by the juices of the cells of the style-tissue 
contiguous to them; and each cell, by its elongation upwards 
and downwards, tends to produce a force which thrusts the 
free end of the pollen-tube along. It may be observed that 
the pollen-tube is in intimate contact with eells throughout 
the whole of its course, and that these cells are as delicate in 
structure as itis. The stimuli to cell growth affect the nu- 
trition of the cells of the style, and these contribute, under 
most favorable circumstances, to the most rapid nutrition and 
consequent elongation of the cells of the pollen-tube. The 
contrary is equally true. The cutting off the pollen-grain, 
and the bisection of the pollen-tube, before its free end has 
even reached half an inch below the line of incision, prove 
the independence of the remaining part of the tube to depend 
