506 MANUAL OF BOTANY 



If we are able from these considerations to recognise in the 

 plant a nervous system in any way comparable to that of the 

 higher animals, we are led to the view that the differences between 

 the two are to be referred to differences of degree of differentia- 

 tion. The latter is on the whole very slight in plants, nothing 

 at all corresponding to the powers of consciousness or volition. 

 There is, however, a differentiation in the sense organs in the 

 direction of sensitiveness, which wiU equal if not surpass the 

 powers of those of an animal. The tendi'il of Passiflora 

 appreciates and vesponds to a pressure which cannot be 

 detected by even the tongue ; the seedlings of Phalaris readily 

 obey the stimulus of an amount of light which is hardly per- 

 ceptible by the human eye. The power of response to the 

 stimulus is of course very much less than in the animal, but 

 this, as we have seen, depends upon the differences in the motor 

 mechanisms. In the vegetable protoplasm we have a much 

 slower response, as well as one of a different kind, the effects 

 taking as a rule longer to manifest themselves, and lasting 

 for a longer time after the stimulus has been withdrawn. We 

 have, however, as in the animal mechanism, a much better 

 response to a cumulative or prolonged stimulation than to one 

 which is rapid and transitory. 



CHAPTEE XVII. 



AUTOMATISM. — RHYTHM. 



The instances of sensibility we have been considering have been 

 seen to be such as are manifested in response to external stimu- 

 lation of various kinds. The responses made have been found 

 usually to take the form of what are generally called movements 

 of either the whole plant or of some of its members. Though 

 called movements, however, they must not be interpreted in all 

 cases to be such in the strict sense. They are rather changes of 

 position brought about by varying turgescence, or alteration of 

 the rate of growth in particular aggregations of cells. The 

 protoplasm has been seen in many instances to play rather a 

 passive than an active part in bringing about the change in 

 position. 



Besides these, however, we have yet to consider some similar 

 phenomena in which the protoplasm is more directly concerned, 

 and the nature of the stimulation, or the exciting cause of the 



