no AN INTRODUCTION TO ZOOLOGY 



it is, manifests itself in a difference of the chemical activities con- 

 cerned with nutrition. The typical plants are able to utilise as food 

 Carbon dioxide, Ammonium compounds and Nitrates, all of which 

 are generally diffused over the surface of the globe in the soil or 

 carried about by the air and the water. From these simple sub- 

 stances, with the aid of radiant energy trapped from the sunlight, 

 the plant is able to build up or synthesise its protoplasm, a method 

 of feeding termed holophytic, or completely plant-like. The typical 

 animal, on the other hand, is unable to do this, and must have a 

 fairly large proportion of proteins in its food. Such substances are 

 not found distributed generally, but only in other living beings, so 

 that the food of an animal must be living or dead organisms ; a mode 

 of feeding termed holozoic, or completely animal-like. The plant 

 can be, and usually is, quite stationary, and is generally modified in 

 such a way as to expose the greatest amount of surface to the air 

 and the water in the soil, and at the same time attain stability. To 

 do this the plant has developed a spreading root to pick up the 

 nitrogenous substances, and a trunk, by means of which a number of 

 flat leaves are exposed to the air, from which they obtain Carbon 

 dioxide. The stability is attained in some measure by the spreading 

 of the root, which acts as an anchor, but largely by the development 

 of a strengthening substance, usually Cellulose, or a closely allied 

 carbohydrate compound. Therein lies one of the most striking 

 differences between plant and animal histology, in the'' plant the cell 

 is practically always enclosed in a moderately thick cellulose wall, 

 while in the animal the cell generally has no cell wall, and cellulose 

 and similar substances are absent from the majority of animals. 

 As the food substances of the plant are already in a drffusable form, 

 there is no need for an elaborate digestive mechanism, but it can 

 at once start to build them up into proteins in most cases by the aid 

 of a green colouring matter, chlorophyll, that is not found in typical 

 animals. 



The animal, on the other hand, has to search for food, and so is 

 modified for motility, and, as a rule, possesses locomotor organs and 

 a body adapted for movement. The food, too, generally consists 

 of solid particles, and not liquids or gases, and so a temporary or 

 permanent aperture is present in the form of a mouth or its equiva- 

 lent. Before the food can be in jested it is frequently necessary for 

 it to be caught, killed or broken up, and so we find a whole series of 

 mechanisms, claws, teeth, etc., to serve these ends. Even when 

 the food is acquired it is not in a form in which it can be assimilated, 

 and so a simple or complex digestive system is provided to make it 

 available for the body. With the food a certain amount of in- 

 digestible matter, supporting structures, etc., is taken in, and has 



