SENSITIVITY IN PLANTS 35 



either increase or diminish the intensity of the original 

 stimulus. 



Occasionally also the effect produced is apparently 

 out of all proportion to the stimulus, as for example 

 the closing of the leaves in a Mimosa or sensitive plant 

 when a few pinnules are touched. When we examine 

 the conditions among living things in which such dis- 

 proportionate results are obtained, we find that we have 

 to deal with a complicated mechanism by which the 

 stimulus is transmitted from cell to cell, and the result 

 is the sum of many separate stimulations. 



The application of a stimulus of any kind is followed 

 by a quiescent period during which no visible response 

 is given, but in which preparations, chemical or physical, 

 or of some sort, are however taking place for the ultimate 

 response. 



The sensitive material in any living organism is proto- 

 plasm, and as there is a great variety of types of proto- 

 plasm, we must expect a very varied set of reactions to 

 stimulation, and this is borne out by daily experience. 



All plants and animals are alike sensitive to stimuli, 

 although the response in animals is more marked and 

 more rapid. A few illustrations of the sensitivity of 

 plants and animals will make this clear. 



If we take a young seedling of suitable size and place 

 it horizontally on damp sawdust, we shall find, on exa- 

 mining it later, that the root has bent downwards 

 towards the moisture and the shoot has bent upwards 

 away from it. Both root and shoot were equally ex- 

 posed to the influence of gravity, but a different response 

 has been given by the two, so that we may lay it down 

 as a general truth, that the same stimulus may produce 



