THE PLANT AND THE ANIMAL 267 



contact with the atmosphere ; but whenever water is 

 substituted for the air, there is no longer the same 

 reflection ; the bodies become more transparent and 

 hence less bright. A direct experiment, however, shows 

 this explanation to be the true one. We have only to 

 make a slight incision on the lower side of the pulvinus 

 of the leaf-stalk to see that a drop exudes from the in- 

 cision at the moment of motion. If a similar incision is 

 made' on a leaf, which has already drooped owing to 

 irritation, the drop of water will not exude. This water, 

 exuded from the cells and occupying the intercellular 

 spaces in the tissue, is absorbed or evaporated in the 

 course of time ; the cells become refilled • with water, 

 and the tissue regains its tension until a subsequent 

 irritation. 



Ultimately, therefore, the cause of the phenomenon 

 which has attracted our attention resolves itself into 

 the fact that water is rapidly exuded from the thin 

 walled cells of the irritable tissue overfilled with it, and 

 consequently this tissue as quickly loses its turgidity. 

 But why is irritation followed by the exudation of water, 

 and what is the energy that forces the cells to be over- 

 filled with water ? We are unable as yet to answer these 

 questions, but very probably we are dealing here with 

 electrical phenomena, as we shall see further on. 



Let us pass on to another example. At the end of 

 the eighteenth century a plant was discovered in the 

 marshes of North America, the movements of which are 

 more striking still. I mean the so-called catchfly 

 (fig. 78) . The upper part of the leaf has the form and 

 function of a trap. Whenever we touch the hairs upon 

 ts surface, or whenever an insect imprudently creeps on 

 to it, the two sides of the trap immediately fold together 

 and do not let their victim out again. The more agitated 

 the entrapped insect becomes, the more tightly do the 

 walls of its prison close. This struggle between the plant 

 and the animal ends always in the death of the animal. 



