SENSITIVENESS 365 



It may well be, however, that they have become impressed 

 upon the organisation of vegetable protoplasm by the con- 

 stant recurrence of these changes of the environment during 

 the long ages of the past. This does not appear unlikely 

 in the face of the fact that, as we shall see later, it is possible 

 under appropriate conditions to impress a new rhythm 

 upon particular organs. The manifestation of rhythmic 

 change has, however, become one of the vital properties of 

 protoplasm. 



We saw in an earlier chapter that the peculiarities of 

 form and structure which different plants possess are to be 

 associated with the character of their environment. From 

 such facts as were there discussed it is evident that a plant 

 is capable of receiving impressions from without and 

 responding to them in various ways. If we examine any 

 plant which does not show such marked adaptation to its 

 surroundings as those which were then more particularly 

 under consideration, we can still find evidence of the pos- 

 session of a similar power of appreciating differences in the 

 external conditions in which it finds itself, and of modifying 

 certain of its vital processes in response. When certain 

 zoospores of some of the lower Algae which swim freely in 

 water are suddenly exposed to a brilliant light, they take 

 up at once a definite position with regard to the incident 

 rays. When a leaf of Mimosa pudica, the so-called sensi- 

 tive plant, is roughly handled, it falls from its normal position 

 and takes up a new one, while its leaflets become folded 

 together. When a filament of Mesocarpus is exposed to 

 an electric shock sent through the water in which it is 

 floating, it is found not infrequently that it splits up into 

 its constituent cells. This power of receiving impressions 

 from without, to which we have had frequently to refer in 

 discussing the phenomena of growth and rhythm, is another 

 property of vegetable protoplasm, and can be observed 

 to belong, in a greater or less degree, to every vegetable 

 organism. It is usually spoken of under the general term 

 irritability or sensitiveness. 



