210 DROSERA ROTUNDIFOLIA. (Car. X. 
showing that the exciting cause of the aggregating process 
has then quite ceased. Exposure to carbonic acid causes both 
the latter process and the motor impulse to travel very slowly 
down the tentacles. We know that the aggregating process 
is delayed in passing through the cell-walls, and we have 
good reason to believe that this holds good with the motor 
impulse; for we can thus understand the different rates of its 
transmission in a longitudinal and transverse line across the 
disc. Under a high power the first sign of aggregation is 
the appearance of a cloud, and soon afterwards of extremely 
fine granules, in the homogeneous purple fluid within the 
cells; and this apparently is due to the union of molecules of 
protoplasm. Now it does not seem an improbable view that 
the same tendency—namely for the molecules to approach 
each other—should be communicated to the inner surface of 
the cell-walls which are in contact with the protoplasm; and 
if so, their molecules would approach each other, and the cell- 
wall would contract. 
To this view it may with truth be objected that when 
leaves are immersed in various strong solutions, or are 
subjected to a heat of above 130° Fahr. (54°°4 Cent.), aggre- 
gation ensues, but there is no movement. Again, various 
acids and some other fluids cause rapid movement, but no 
aggregation, or only of an abnormal nature, or only after a 
long interval of time; but as most of these fluids are more or 
less injurious, they may check or prevent the aggregating 
process by injuring or killing the protoplasm. There is 
another and more important difference in the two processes; 
when the glands on the disc are excited, they transmit some 
influence up the surrounding tentacles, which acts on the 
cells at the bending place, but does not induce aggregation 
until it has reached the glands; these then send back some 
other influence, causing the protoplasm to aggregate first in 
the upper and then in the lower cells. 
The Re-expansion of the Tentacles. -This movement is always. 
slow and gradual. When the centre of the leaf is excited, or 
a leaf is immersed in a proper solution, all the tentacles bend 
directly towards the centre, and afterwards directly back 
from it. But when the point of excitement is on one side of 
the disc, the surrounding tentacles bend towards it, and 
therefore obliquely with respect to their normal direction ; 
when they afterwards re-expand, they bend obliquely back, 
so as to recover their original positions. The tentacles 
