INTRODUCTION 



combination which is called a proteid ; they can only take their carbon 

 either from a proteid or from a carbohydrate or a hydrocarbon. 



These elaborate compounds only occur in the bodies of other 

 animals or of plants. Hence animals absolutely depend for their 

 food on other living things. Plants, on the contrary, are (with 

 certain exceptions) able to take up as food the compounds of carbon 

 and of nitrogen which may be called the stable or resting condition 

 of those elements namely, the simple oxide of carbon carbonic 

 acid gas and the simple compound of nitrogen with hydrogen which 

 is called ammonia, or the oxide of nitrogen which forms nitrates. 

 This " food " of plants is diffused throughout the earth's surface in 

 air and water ; hence they need to expose a large absorbing surface 

 to those media ; hence their branches and leaves spread in tree- 

 like form to the air or to the water, whilst their roots are spread to 

 the water contained in the soil. Their food is ever moving and 

 flowing around them : they have neither to move in search of it 

 nor to seize it. Hence the majority of plants are fixed and find 

 safety and protection in stability. Animals, on the other hand, have 

 to obtain their food from the scattered, solid, separate bodies of 

 plants or of other animals. They have to move in search of it, they 

 have to seize it when found, and they have to act chemically on 

 the solid or viscous body or fragment of their prey so as to dis- 

 solve it and to enable the dissolved material containing the precious 

 carbon and nitrogen in a high state of chemical combination to 

 diffuse into their living substance and there be further assimilated 

 and built up into the material of protoplasm. For these purposes 

 animals possess structures enabling them to move more or less 

 rapidly, and others enabling them to seize or grasp. Further, and 

 of even more fundamental a character as determining their whole 

 shape and organisation, they possess (with rare and intelligible 

 exceptions) an aperture, the mouth, leading into a relatively extensive 

 cavity, the gut, into which the solid or viscous mass of food is intro- 

 duced, and when there is chemically dissolved or " digested." 



The obvious and predominant difference in the make and habit 

 of plants as compared with animals is thus connected with the very 

 great and definite difference in the nature of the food of the two 

 groups. 



These statements are true in a general way, but require 

 qualification. In the first place, we find it necessary to regard as 

 genetically part of the great Plant series many organisms which are 

 not able to procure their carbon from carbonic acid nor their nitro- 

 gen from ammonia. Only the green plants are able to perform this 

 constructive feat. The protoplasm of the more superficial cells of 

 green plants contains corpuscles impregnated with a transparent 

 green matter known as chlorophyll. In the presence of and in 

 virtue of the physical action of sunlight screened by their chloro- 



