60 FOUNDATIONS OF BIOLOGY 



tissues which has assumed a definite form for the perform- 

 ance of a certain function: for example, the human hand 

 composed of bone, muscle, nerve, etc.; or the plant leaf with 

 its chlorophyll-bearing tissue, epidermal covering, etc. 



As one would naturally expect, among the lowest Meta- 

 phyta and Metazoa there are forms in which the body is 

 relatively simple, without highly specialized tissues and 

 organs, but in most animals specialization is carried still 

 another step forward by the grouping of organs devoted to 



FIG. 21. Portion of a cross section of a young plant stem, magnified 

 to show cellular differentiation and tissues. ca, cambium; co, cortex; e, 

 epidermis; p, pith; ph, phloem; x, xylem. (From Gager.) 



the performance of some one general function into an ORGAN 

 SYSTEM. An animal has many muscles, each of which is an 

 organ but which collectively constitute a working unit, the 

 muscular system; or it has stomach, intestine, liver, etc., form- 

 ing the digestive system. On the other hand, even in the 

 highest plants, differentiation has proceeded neither in just 

 the same way, nor so far, since the body is composed of 

 TISSUE SYSTEMS rather than organ systems. This point will 

 be clear when the structure of the plant and animal body 

 has been considered. 



