6 INTRODUCTION 



then as many cells as there are nuclei, and, frequently, each 

 cell is marked off from its neighbors by a wall of inert substance 

 secreted by the protoplasm. The character of this wall varies, 

 not only with the kind of organism, but also with the organ in 

 which it is found. In the central part of a tree trunk the thick 

 cell walls form the firm substance of wood; similarly, near the 

 surface they form the corky layers of the bark. 



17. What has been said in the preceding paragraphs has 

 a general application to all organisms, but the obvious grouping 

 of living things into two kingdoms — the vegetable and the 

 animal — is based on certain distinctive peculiarities which are 

 of such far-reaching significance and which separate the more 

 familiar forms of the two groups so precisely that it will be con- 

 venient to study each group separately. Plants are on the 

 whole much simpler than animals and therefore better adapted 

 for introductory study. 



