INTRODUCTOKY. 3 



food, contained in the air, and liquid food, present in the earth. 

 These are sucked in or absorbed by the general surface of the 

 body, the area of which is much increased by the freely-branched 

 or diffuse form so characteristic of plants. Animals, on the other 

 hand, utilize a great deal of solid food, which is usually taken in 

 by a mouth, and received into a digestive cavity, where it is, to a 

 greater or less extent, brought into a state of solution, or else of 

 fine division. The shape of animals is compact, in accordance 

 with the solid nature of their food. 



(2.) Again, plants, at any rate green plants, live on very simple 

 food, namely, the carbon dioxide or carbonic acid (C0 o ) of the air, 

 and watery solutions of mineral substances (salts) contained in the 

 soil These they can build up into the complex substances com- 

 posing their own bodies. Animals require complex food, derived 

 from plants or other animals. 



(3.) The nature of the food also exerts an influence upon the 

 plant or animal in another direction. The air and earth are full 

 of plant-food, and, by extending branches of the stem and root 

 into them, a tree or herb can obtain an abundant supply. Hence 

 powers of spontaneous locomotion are not possessed by plants. 

 The complex food of animals is less uniformly distributed, and 

 conspicuous powers of locomotion are generally possessed by them, 

 one main aim being to find and secure suitable food. 



(4.) Higher animals are also characterized by the possession of 

 a nervous system, i.e., organs for regulating the body generally 

 and rendering it sensitive to external influences. There are central 

 organs (brain and the like) exerting control, and these are placed 

 in communication with all parts of the body by definite strands 

 or nerves, along which impulses pass, and the sole use of which is 

 to convey such impulses. No such arrangement is found in any 

 known plant, although a local sensitiveness is sometimes exhibited 

 (e.g., sensitive plant). 



(5.) It may be mentioned as a further point, that plant-hairs 

 and membranes are largely composed of a complex substance, 

 cellulose, allied to starch and composed of carbon, oxygen, afid 

 hydrogen. Cotton is a very pure form of this body. In the 

 animal kingdom cellulose is mainly conspicuous by its absence. 



Exceptions. — The preceding tests are not absolute, even among 

 the higher plants. Some few are not green (e.g., clover-dodder), 

 and these, like animals, require complex food, though this is not 

 taken into the body as solid particles. Such forms are termed 

 parasites when they prey upon living organisms, saproplnjf<x 

 when they subsist on complex compounds derived from the dead 

 bodies of plants or animals. The " insectivorous " or " carni- 

 vorous " plants, again, partly live on flies and the like, parts of 



