ECOLOGICAL 163 



blade rapidly close, the movement being due to a sudden collapse 

 of cells along the midrib. But a slower closure may also be provoked 

 by stroking or pressing the surface of the leaf with a splinter of wood, 

 or by a nitrogenous chemical stimulus on the same surface. In 

 ordinary cases the stimulus is due to the insect's feet touching the 

 side of a sensitive hair; and it is interestingly adaptive that a drop 

 of rain falling vertically on a sensitive hair has no effect. According 

 to Macfarlane, mechanical stimulus of Dionaea requires two touches, 

 unless the touch be very powerful ; and the touches must be separated 

 by an interval of more than a third of a second. If less than that 

 interval be allowed, no contraction follows, and a third stimulus is 

 then necessary. A neat adaptation is the joint near the base of the 

 hair, where there is a slight constriction, and above that a narrow 

 zone of joint-cells which elongate at one side and shorten at the 

 other, thus enabling the hair to fold down. If it were not for this 

 flexible base, the six hairs would be crushed when the trap closed. 



If an insect is caught, there follows a copious secretion of digestive 

 ferment (plus a little formic acid) from glandular cells, which form 

 the attractive rosy patches on the surface of the leaf -blade. This 

 chemical stimulus induces a stronger closure of the trap, so that the 

 insect is squeezed against both leaf-blades. Here again there is 

 adaptation, for the introduction of a non-digestible body does not 

 evoke the secretion, and in the absence of the chemical stimulus 

 there is no second contraction. In normal circumstances a leaf opens 

 (juickly if it has closed on a non-digestible body, but if an insect has 

 been captured the trap remains shut for a week or so, during which 

 the booty is digested and absorbed. If the insect that is captured be 

 too large, the trap never reopens; and even in natural conditions 

 a leaf is not able for more than two or three meals in the course of 

 its life. Although the movement of the Flytrap is very different 

 from that of muscle, Burdon Sanderson showed that there is a 

 similar electrical change in both cases; and it is possible that the 

 electrical change is also correlated with the secretion, for so it is in 

 the case of many animal glands. Here again Bose's later work 

 should be referred to. 



Experiments with this "carnivorous vegetable", as Bartram 

 called it in 1790, have shown that if it is cheated into closing by 

 inappropriate mechanical stimulus, it soon opens again ; and that a 

 repeated cheating induces a state of insusceptibility, during which 

 it fails to respond to a touch which would normally evoke move- 

 ment. But this does not last long; the "vegetable memory" is 

 short; and after a rest the Dionaea may be cheated again. 



Allied to Dionaea is a rootless water flytrap, Aldrovandia vesi- 

 culosa, which lives in well-sunned pools in south and central Europe 

 and also in Austraha and India. There are whorls of 8-9 little leaves, 

 somewhat like miniatures of the Dionaea traps. The two halves of 



