Io PLANTS AND ANIMALS. 
general way that the insoluble proteids are converted into soluble 
peptones, insoluble carbohydrates into sugars, and fats into soaps and 
glycerine, though in some cases the fats are emulsified or broken into 
minute particles which are then carried into the organism. 
The next important point to notice is the constitution of 
foods. Leaving out of consideration the minerals, which 
are only of secondary importance, we find that the simplest 
animal-foods are complex compounds of carbon, hydrogen 
and oxygen, and that others have these elements with the 
addition of nitrogen, sulphur and phosphorus. An animal is 
incapable of building up its protoplasm from any simpler 
products. It would be easy to supply an animal with mineral 
salts alone, such as nitrates, sulphates and carbonates, con- 
taining all the chemical elements in protoplasm, but they 
would be of no practical use to the animal in the formation 
of fresh protoplasm. 
Plants and Animals.—On the other hand, it is 
typical of plants that they can build up protoplasm from 
such simple compounds as carbonic acid, water and mineral 
salts, all of which are soluble and diffusible, either as gas 
or liquid. 
Hence the primary distinction between a plant and an 
animal rests in the power of the former to perform the 
synthesis of compounds containing carbon, hydrogen and 
oxygen from carbonic acid and water. This power resides 
in the presence of chlorophyll, a green colouring matter, 
which under suitable conditions of warmth and sunlight 
can effect the important synthesis. From this we can 
derive the other differences between animals and plants. 
The food of animals being solid, they require digestive 
organs to bring it into a condition suitable for absorption. 
Again, they require motor organs, for solids of this nature 
are in isolated masses (plants and other animals) and must 
be sought for. 
The liquid and gaseous food of plants being already in 
a condition for absorption (or assimilation) no alimentary 
organs are required, and, being universally distributed, there 
is no necessity for movement; the absence of movement 
implies a low condition of the function of sensation. 
We have already referred to the relationship between 
the surface and the bulk of an animal, and in a typical 
