Cuar. XL] GENERAL SUMMARY. 213 
It was shown in the second chapter that animal substances 
placed on the discs cause much more prompt and energetic 
inflection than do inorganic bodies of the same size, or mere 
mechanical irritation; but there is a still more marked 
difference in the greater length of time during which the 
tentacles remain inflected over bodies yielding soluble and 
nutritious matter, than over those which do not yield such 
matter. Extremely minute particles of glass, cinders, hair, 
thread, precipitated chalk, &c., when placed on the glands of 
the outer tentacles, cause them to bend. A particle, unless 
it sinks through the secretion and actually touches the 
surface of the gland with some one point, does not produce 
any effect. A little bit of thin human hair ,,8,5 of an inch 
‘203 mm.) in length, and weighing only 7s4,, of a grain 
‘000822 mg.), though largely supported by the dense 
secretion, suffices to induce movement. It is not probable 
that the pressure in this case could have amounted to 
that from the millionth of a grain. Even smaller particles 
cause a slight movement, as could be seen through a lens. 
Larger particles than those of which the measurements 
have been given cause no sensation when placed on the 
tongue, one of the most sensitive parts of the human 
body. 
ee ensues if a gland is momentarily touched three 
or four times; but if touched only once or twice, though with 
considerable force and with a hard object, the tentacle does 
not bend. The plant is thus saved from much useless 
movement, as during a high wind the glands can hardly 
escape being occasionally brushed by the leaves of sur- 
rounding plants. Though insensible to a single touch, the 
are exquisitely sensitive, as just stated, to the slightest 
pressure if prolonged for a few seconds; and this capacity is 
manifestly of service to the plant in capturing small insects. 
Even gnats, if they rest on the glands with their delicate 
feet, are quickly and securely embraced. The glands are 
insensible to the weight and repeated blows of drops of 
heavy rain, and the plants are thus likewise saved from much 
useless movement. 
The description of the movements of the tentacles was 
interrupted in the third chapter for the sake of describing 
the process of aggregation. This process always commences 
in the cells of the glands, the contents of which first become 
