380 VEGETABLE IRRITABILITY. 



(Sundews) show marked irritability. The leaf of Dionsea muscipula 

 (Veaus's Ply-trap), a plant of that order, exhibits movements when 

 touched. The leaf represented in figure 660 is composed of a pyUoid 



petiole, 'p, and a lamina, I, consist- 

 ing of two movable halves united 

 by a strong midrib. The lobes 

 when open are placed at right angles 

 to each other. Along the edge of 

 each of them there are about 

 twenty spiny hairs ; on the reddish 

 upper surface there are numerous 

 glands and three stiff hairs, a, with 

 glandular bases on each division 

 of the lamina. These, when touch- 

 ed, cause the leaf to close with 

 Jig. 660. considerable force. An insect alight- 



ing on these hairs is instantly 

 entrapped. If it is very small it may escape through the grating 

 formed by the crossing of the teeth, and the leaf will soon open 

 again ; but if it is large it cannot get out, as the two halves of the 

 lamina close firmly on it, and the spines at this edge interlace more 

 and morp completely, like the teeth of a rat-trap, the irritation being 

 kept up by the struggles of the insect. If the hairs are touched with 

 a camel-hair pencil the leaf closes, and may remain so for twenty- 

 four hours. Such is also the case if a small piece of any mineral and 

 indigestible matter is placed in the leaf But if a fiy is caught] then 

 the closure continues for a week or more. During the progress of 

 the pressure the spiny hairs at the margin lose their interlacing posi- 

 tion and become more or less erect, and finally the lamina opens. 

 There is often a bulging on the outside of the lobes caused by the 

 body of the insect contained within them. 



During the , time of closure the glands pour forth a peculiar 

 secretion of an acid nature, as shown by the effect on litmus paper. 

 This secretion acts upon the insect, which is gradually digested, 

 nothing being left ultimately but the dry outer covering of the animal. 

 The same thing occurs when a very small piece of flesh is grasped by 

 the leaf. The leaf closes on it, and it is so completely digested that 

 when the leaf opens there is nothing left. If too large a quantity of 

 flesh is inserted digestion is not carried on properly, and th.e lamina 



Fig 660. Leaves of Veniis's Fly-trap {Iiwrwsxi muscipula), which exhibit evident irrita^ 

 bility. The leaf consists of two parts, a lamina or blade, I, and a petiole or leafstalk, p. 

 The two halves of the blade are united by a sort of hinge, a, and there are on each of them 

 three hairs, which, when touched, cause the folding of the lamina in the way represented 

 at I and b. At the base of each of the hairs there is a swelling. The irritation seems to be 

 communicated by means of the vessels to the midrib, and the folding is owing to the tur- 

 gescence of the lower cells of the midrib. The motion is of the nature of a hinge-joint. 



