IRRITABILITY OF PLANTS. 23 



but slight movements not more intense than those of plants. This 

 irritability, however, appears widely present among the higher plants. 

 The sensitive plants move their leaves on the application of mechani- 

 cal stimuli (Jfimosece), or bend like the sundew (Drosera, fig. 7) 

 small knobbed processes of the leaf surface which are comparable to 

 the tentacles of polyps. The fly-catcher (Dioncea, fig. 8) brings the 

 two halves of the leaf together in a valve-like manner when touched 

 by insects. The stamens of the Centaurea contract along their whole 

 length on mechanical and electrical stimulation, and according to the 

 same laws as do the muscle of the higher animals. Many flowers 

 open and shut under the influence of light at cerbain times of the 

 day. 



Accordingly irritability as well as contractility appears to be a 

 property both of vegetable tissue and of the protoplasm of vegetable 

 cells ; and it is not possible to determine whether volition and 

 sensation, which we exclude from these phenomena in plants, play a 

 part in the similar sensory and motor phenomena of the lower 

 animals. 



In none of the above-mentioned characteristics of animal and 

 vegetable life, then, do we find any absolute test, and we are not in 

 a position to indicate the presence of a sharp line between the two 

 kingdoms. 



From the common starting-point of the contractile substance* 

 animals and plants are developed in different directions ; at the 

 beginning of their development they present many kinds of resem- 

 blance, and it is only on their attaining a more complete organization 

 that the full opposition between them is apparent. In this sense, 

 without wishing to draw a sharp line between the two series of 

 organization, we can define our conception of an animal by putting 

 together all the characteristics distinguishing the direction of animal 

 development. 



An animal, therefore, is to be defined as an organism provided 

 with the power of free and voluntary movement, and with sensation ; 

 whose organs are internal, and are derived from a development of 

 the internal surfaces of the body ; which needs organic food, inspires 

 oxygen, changes potential energy into kinetic under the influence of 

 oxidation processes in metabolism, and excretes carbonic acid and 

 nitrogenous waste productr. 



* The formation of an intermediate kingdom for the simplest forms of life 

 is neither scientifically justified, nor from practical considerations desirable. 

 On the contrary, the acceptance of the Protista would only double the difficulty 

 of determining the limit. 



