VI 



CELLS AND TISSUES 



Preview. Living things composed of cells • Plant and animal cells 

 differ in size, shape, and structure • Why cells divide • How plant cells 

 divide • How animal cells divide • Tissues • The tissues in plants; the 

 meristematic tissues ; the protective tissues ; the fundamental tissues ; the 

 conducting tissues ; the tissues in animals ; the epithelial tissues ; the sup- 

 porting tissues ; the muscular tissues ; circulatory tissue ; the nervous tissues ; 

 reproductive tissues • Why are living organisms so called? • Suggested 

 readings. 



PREVIEW 



One characteristic of living things is that they are organized into 

 tiny units of Uving matter which have been called, rather inaptly, 

 "cells," because an Englishman, Robert Hooke, as early as 1665, 

 described the construction of cork which he saw under a lens as 

 "little boxes or cells distinguished from one another." He cut 

 cross sections with a penknife and saw that they were "all cellular or 

 porous in the manner of a honeycomb, but not so regular." What 

 Hooke saw was the woody walls enclosing spaces which in younger 

 plants would be filled with living matter. 



From a comparison with the simplest organisms, it is evident that 

 the more complex forms are built up of cells, and that, although each 

 cell can function as an organic whole, far more efficient results are 

 obtained when groups of cells organized into tissues do the work. The 

 consideration of groups of cells, according to their structure and func- 

 tion, constitutes in itself a major chapter in biological study, called 

 Histology. The study of individual cells, which make up the sub- 

 ject matter of Cytology, is absolutely indispensable to a proper 

 understanding of the organism as a whole. 



The problems for reading and further study are so numerous that 

 we might spend the major part of our available time in discussing 

 them. Why and how do cells divide? What are the differences 

 between plant and animal cells? What are the reasons for having 

 tissues and organs? How did many-celled organisms come into 

 existence, and why? The pages which follow will enable the student 

 to make at least a start on some of these interesting questions. 



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