214 DROSERA ROTUNDIFOLIA. [Cuap. XI. 



It was shown in the second chapter that animal sub- 

 stances placed on the discs cause much more prompt and en- 

 ergetic inflection than do inorganic bodies of the same size, 

 or mere mechanical irritation; but there is 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 tAt5 of 

 an inch (.203 mm.) in length, and weighing only rF7T5 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. 



Movement 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 

 surrounding plants. Though insensible to a single touch, 

 they 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 in- 

 sensible to the weight and repeated blows of drops of heavy 

 rain, and the plants are thus likewise saved from much iise- 

 Icss 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 



