INTRODUCTORY. 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 (CO,) of the air, 
and watery solutions of mineral substances (sa/ts) 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, 2.€., 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, and 
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, saprophytes 
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 lke, parts of 
