CHAP. L] OF A NERVOUS SYSTEM. 3 



decompose water, so as to retain its hydrogen ; whilst 

 they also abstract nitrogen either directly from the 

 atmosphere, or indirectly from the nitrate of ammonia 

 formed therein and brought to the soil in refreshing 

 showers. This work of decomposition, under the in- 

 fluence of light and heat, goes hand in hand with one of 

 an opposite kind, resulting in the elaboration of those 

 organic and living compounds which enter into the com- 

 position of vegetal tissues. 



(2.) Then again, as a rule, plants exhibit no inherent 

 powers of movement other than those connected with 

 their growth. The movements of the Sunflower and its 

 allies are exceptional ; and there are very few plants 

 which more or less immediately respond to a touch by a 

 movement, in the way that the Sensitive-plant or the 

 Venus fly-trap is known to do. To this subject, however, 

 and to the causes of such motions in plants, it will be 

 necessary to return. For the present it is of importance 

 to recollect that plants do not move at all in search of 

 food. 



The comparative simplicity of the life-processes of 

 plants is in the main due to these two peculiarities. They 

 are also, perhaps, the most fundamental attributes of 

 plants as distinguished from animals. This subject is 

 well worthy of our brief attention, since if its considera- 

 tion should lead us to anything like a correct apprecia- 

 tion of the mode in which some of the simplest vegetal 

 organisms differ from some of the simplest animal organ- 

 isms, this insight may apart from its own intrinsic 

 interest prove of the highest importance in regard to 

 our present inquiry. It may enable us, in a measure, to 

 comprehend why a Nervous System is absent from Plants, 

 and why it comes into existence in Animals. It may help 

 us further to comprehend why this nerve tissue gradually 



B 2 



