PLANT PHYSIOLOGY 137 



is not locomotion. In addition to this, some plants have the 

 peculiar property of closing their leaves in the night. The leaves 

 droop and close themselves in such a way as to present a small 

 surface for evaporation. This motion is sometimes spoken of 

 as the sleep of plants. It is not developed in all, but it is more 

 common than has generally been believed. 



Thus, while it is believed that plants do not as a rule possess 

 the power of motion and, except in the lowest forms, no power 

 of locomotion, it is not absolutely true that motion is lacking 

 in the vegetable kingdom. Speaking in general, however, plants 

 are characterized by absence of motility. 



Coordinating Functions. Plants have nothing whatever that 

 corresponds to a nervous system in the sense of possessing nerves 

 or nerve fibers which coordinate the different parts of the body. 

 There is practically no coordination between the functions carried 

 on in the different parts of the plant. True sensory functions are 

 also lacking from plants. In a general way the protoplasm of 

 plants, as well as that of animals, is sensitive. All protoplasm 

 reacts under certain stimuli and is therefore sensitive. Moreover, 

 there are some of the higher plants which react so quickly and so 

 strongly to certain stimuli that they are spoken of as sensitive 

 plants. In the common so-called sensitive plant a touch upon 

 the leaf will cause the leaf to close, and a slight touch of the 

 branch will cause all the leaves on that branch to droop. Such a 

 condition, however, is very unusual among plants, and in these 

 cases it is incorrect to speak of the plants as sensitive in any 

 proper" sense. There is no reason for thinking that the plant has 

 any sensation, i.e., any true consciousness; and all that is meant 

 by being sensitive in these cases is a quick ability to respond to 

 an external stimulus. 



