62 Professor R. H. Yapp on The Sense Organs of Plants. 



Apparently it is merely the intensity of the light focussed in the 

 cells which is important.* 



We have travelled somewhat widely to night over the field of 

 movement in plants. Enough has been said to show that plant-s, 

 like animals, have a remarkable power of receiving impressions 

 from the outside world, and of utilising these impressions for their 

 own ends. I have laid emphasis on the similarities, rather than 

 the differences, between the two great divisions of the organic 

 world. We have seen that the sense organs of animals have, to 

 some extent, their counterparts among the higher plants. But we 

 must be careful not to press our analogies between the two 

 kingdoms too iar ; and it must be pointed out that no plant 

 possesses the central nervous system so characteristic of higher 

 animals. However, the fact stands out clearly, that though the 

 elaborate mechanism of an animal, regarded as a whole, may differ 

 widely from that of a plant, the properties of rhe living protoplasm 

 of the two are fundamentally the same. 



The deeper possibilities which underHe the phenomena 

 described to-night I am not competent even to discuss. Are we 

 to see, for example, as some do, in the powers of perception 

 undoubtedly possessed by plants, the dim beginnings of the dawn 

 of consciousness ; or of definite psychological processes, even 

 though far removed from those of man and the higher animals ? 

 Fascinating though this and similar questions may be, they are 

 problems for the philosopher rather than the botanist. I will only 

 suggest, in concluding, that the behaviour of different plant organs 

 towards light, gravity and other stimuli, is to a certain extent a 

 matter of habit. Plants are historic beings, and the habits of 

 response now so deeply ingrained in the very nature of the plant 

 are habits acquired, that is, lessons learned from experience, long 

 ages ago, by the ancestors of the plants of to-day. And, looked 

 at from this point of view, what is habit but a kind of unconscious 

 memory? 



* Since this Lecture was delivered I have become aware of some as yet unpublished 

 researches which appear to throw doubt on the theory outlined above. TV^ facts are for the 

 most part as stated, and the epidermal cells do undoubtedly act as lenses. B'or the moment, 

 however, judgment must be suspended on the point as to whether or not these lens-cells are 

 really the light perceiving organs of the leaf. 



