TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS. 41 



kingdom. The familiar members of the first kingdom seem to us 

 to differ in nearly every important respect from the best known 

 members of the second; an oak tree and a horse are superficially 

 wholly dissimilar. Yet both plants and animals consist of living 

 cells, and the protoplasm of which plant cells are composed is often 

 indistinguishable from that found in animal cells. When we at- 

 tempt to analyze the difference between plants and animals we find 

 that it cannot be referred to difference in the protoplasm. The 

 simplest plants and the simplest animals show the fundamental 

 properties of protoplasm developed in about equal degrees. In 

 fact it is by no means easy always to state positively whether a 

 given one-celled organism should be considered a plant or an 

 animal. 



As we go up the scale to the region of higher organization, how- 

 ever, we find no difficulty in deciding whether a living form is plant 

 oj animal. The fundamental difference between the higher plants 

 and animals, and the one which involves, as a natural sequence, 

 the superficial differences which are so striking, is a difference in 

 the manner of obtaining nourishment. The higher (green) plants 

 are able to use the energy which falls upon them in the form of 

 sunlight to build up from simple substances present in the air, and 

 in the water to which their roots penetrate, the complex materials 

 of which protoplasm is composed and which, through the processes 

 of dissimilation, provide for the carrying on of the necessary ac- 

 tivities of living cells. The higher animals, on the other hand, are 

 nourished by means of complex materials which contain within 

 themselves the energy required for the bodily activities. The 

 ultimate source of animal energy, to be sure, is the same as of 

 plant, for the complex materials consumed by animals are derived, 

 directly or indirectly, from plants. 



The simple chemical materials needed by plants are very widely 

 distributed, and the sunlight falls, of course, on all parts of the 

 earth. In any location, therefore, that is sufficiently suited to 

 plant life to allow the plant to get a start, the chances of being able 

 to continue to live are as good, on the whole, as they would be 

 anywhere. Hence plants do not need to move about. The giant 

 sequoias of the Pacific Slope have lived for centuries upon the 

 spots where they became established as seedlings. 



Animals, on the other hand, require materials that are not every- 



