26 INTRODUCTORY. PLANTS AND ANIMALS 



substance and store energy. And since life consists in the 

 expenditure of energy, animals live much more intensely 

 than plants, and correspondingly they are sensitive to 

 much more varied stimuli, i.e. to the influences which 

 lead to their expending energy in definite ways. But 

 because they are alive plants do spend energy and are 

 sensitive to stimuli. In the course of the life of a tree, 

 for instance, a large amount of energy is expended in 

 lifting the branches high into the air and in pushing 

 the root tips through the soil, though these things 

 are done so slowly, measured by the standard of the 

 rate of motion in animals, that we are not at once 

 impressed by them. The shoots of a plant also bend 

 towards the light, and the roots towards supplies of 

 water, i.e. they respond to different stimuli in definite 

 ways. Also animals do store energy in various forms 

 of organic food, e.g. glycogen (a form of starch) in the 

 liver and the muscles, and thus have a reserve supply 

 of potential energy which enables them to carry on 

 their active life for a time without consuming fresh food. 

 The Naming and Classification of Plants and Animals. 

 The immense multitude of individual plants and 

 animals existing on the earth are exceedingly varied 

 in form and structure, and in order to get any under- 

 standing of plant or animal life we must classify them 

 in some way. We recognise at once mankind has 

 recognised from the earliest times that there are 

 many different kinds, but of these some resemble each 

 other so closely that they are difficult to distinguish, 

 and can only be separated by those who have made 

 a special study of the forms in question, while others 

 are very distinct indeed. For instance, a blackberry 

 is obviously different from a raspberry in the colour 

 and taste of the fruit, and in the fact that the former 



