INSECTIVOROUS PLANTS. 51 



margin project in the same plane with the leaf, or more commonly 

 {see Fig. 2) are considerably reflexed. 



If a small object be placed on the glands in the centre of the leaf, 

 a motor impulse is transmitted tQ the marginal tentacles. The nearer 

 ones are first affected, and then those farther off, until at last all are 

 slowly but unerringly inflected, and close over the object. This takes 

 place in from one to five or more hours ; the difference in time de- 



FiG. 2. Drosera eotundepolia. Old leaf viewed laterally ; enlarged aboat five times. 



pending on several circumstances, as the size of the object and its 

 nature; on the vigor and age of the leaf; w^hether it has lately been 

 in action ; and the temperature. 



The tentacles in the centre do not become inflected when directly 

 e:jcited, though they are capable of inflection if excited by a motor 

 impulse from other glands ; but through and from them the motor 

 impulse spreads gradually on all sides. Such is not the case with the 

 marginal tentacles. If a bit of meat be placed on one of these it 

 quickly transmits an impulse to its own bending portion, but never 

 to those adjoining {see Fig. 5), for these are never affected until the 

 meat has been carried to the central glands, which then radiate their 

 conjoined impulse on all sides. 



The sensitiveness of the leaves is located in the glands together 

 with the immediately underlying cells of the tentacles. Though it is 

 necessary that the glands should be touched, it is wonderful how 

 slight a pressure will sufiice. A bit of human hair -^^ of an inch in 

 length and weighing only , g ^ 4 ^ of a grain will induce motion, trans- 

 mit a motor impulse through the whole length of a marginal ten- 

 tacle, and cause it to sweep through an angle of 180 or more. This 

 minute morsel, it must be borne in mind, rests upon and is supported 

 by the dense, viscid fluid which surrounds the gland, and the pressure 

 is thus rendered inconceivably slight. Mr. Darwin conjectures that 

 it may be less than the millionth of a grain. While the pressure 

 may be extremely slight, it needs must be steady. A shai-p, sudden 

 brush of the tentacles does not induce inflection, nor do drops of 

 water falling upon the glands from any height. This specialized na- 

 ture of the sensitiveness may readily be seen to be of gi'eat use to the 

 plant, effecting an economy of time and energy, for the process of 

 inflection is slow and that of reexpansion still slower, often occupy- 



