162 ELEMENTS OP STRUCTURAL BOTANY. 



are due to the lengthening and hardening of the cells 

 of which they are composed (Fig 221). 



265. The Oell-wall. In the portions of plants just 

 selected for microscopic examination we have seen that 

 the protoplasm is in every instance bounded by a wall. 

 It has been ascertained that the wall is a chemical com- 

 pound of carbon, hydrogen, and oxygen, and to this 

 compound the name cellulose has been given. 



We have said that the protoplasm is the active principle 

 through the agency of which all the vital processes of 

 the plant are carried on. It contains at some time or 

 other every constituent of the plant. The cell-wall is 

 itself, therefore, a product or secretion of the protoplasm, 

 and is at first an extremely thin film, which, however, 

 gradually increases in thickness by the addition of further 

 material This new material is deposited between the 

 molecules of the original film, and so extends not only 

 bhe surface of the wall, but, by deeper deposits, the 

 bhickness also. This process of acquisition of new 

 material is known as intussusception. 



266. As the wall between two cells increases in thick- 

 ness, a distinct middle layer is discernible in it, known as 

 the middle lamella. This portion of the common wall is 

 different in chemical composition from the rest, so that it 

 may, under proper treatment, be dissolved and the cells 

 thereby separated. 



267. It is in the earlier stages of their history, while 

 the walls are comparatively thin, that the cells possess 

 the greatest activity. By these alone is carried on the 

 process of growth, which consists in the multiplication 

 and enlargement of cells. 



