Crar. X] SENSITIVENESS OF THE LEAVES. 187 
CHAPTER X. 
ON THE SENSITIVENESS OF THE LEAVES, AND ON THE LINES OF 
TRANSMISSION OF THE MOTOR IMPULSE. 
Glands and summits of the tentacles alone sensitive—Transmission of the 
motor impulse down the pedicels of the tentacles, and across the blade of 
the leaf—Aggregation of the protoplasm, a reflex action—First discharge 
of the motor impulse sudden—Direction of the movements of the tentacles 
—Motor impulse transmitted through the cellular tissue—Mechanism of 
the movements—Nature of the motor impulse—Re-expansion of the 
tentacles. 
We have seen in the previous chapters that many widely 
different stimulants, mechanical and chemical, excite the 
movement of the tentacles, as well as of the blade of the leaf; 
and we must now consider, firstly, what are the points which 
are irritable or sensitive, and secondly how the motor impulse 
is transmitted from one point to another. The glands are 
almost exclusively the seat of irritability, yet this irritability 
must extend for a very short distance below them; for when 
they were cut off with a sharp pair of scissors without being 
themselves touched, the tentacles often became inflected. 
These headless tentacles frequently re-expanded ; and when 
afterwards drops of the two most powerful known stimulants 
were placed on the cut-off ends, no effect was produced. 
Nevertheless these headless tentacles are capable of sub- 
sequent inflection if excited by an impulse sent from the disc. 
I succeeded on several occasions in crushing glands between 
fine pincers, but this did not excite any movement; nor did 
raw meat and salts of ammonia, when placed on such crushed 
glands. It is probable that they were killed so instantly 
that they were not able to transmit any motor impulse; for 
in six observed cases (in two of which, however, the gland 
was quite pinched off) the protoplasm within the cells of the 
tentacles did not become aggregated ; whereas in some 
adjoining tentacles, which were inflected from having been 
roughly touched by the pincers, it was well aggregated. In 
like manner the protoplasm does not become aggregated 
when a leaf is instantly killed by being dipped into boiling 
