CHAPTER VI 

 THE CELL WALLS OF PLANTS 



The tissue cells of plants, like those of animals, are separated by 

 intercellular substance. In both cases each cell has a delicate plasma 

 membrane, but in plants each has in addition a relativel}^ firm wall 

 between the membrane and the intercellular substance. Such cell walls 

 vary greatly in degree of development and structural complexity. Their 

 chief constituent is cellulose, and with this other materials are usually 

 associated. Surely nobody needs to be reminded of the importance of 

 cellulose as supporting material in large plants or of the varied roles 

 played by this substance in our modern life. 



EesSii^ 



Fig. 51. — Diagrammatic repre.sentation of successive stages (left to right) in formation 

 of plant cell wall with secondary thickening, a, origin of cell plate; b, cell plate transform- 

 ing into middle lamella, or intercellular substance {ml) ; c, beginning of deposition of 

 primary wall layers (1, 1); d, e, f, beginning of deposition of outer, middle, and inner 

 portions (o, m, i) of secondary wall layers (2, 2); g, completion of wall thickening; pm, 

 plasma membrane; cyt, cytoplasm. {Based on researches of W. A. Becker, I. W. Bailey, 

 T. Kerr, and others.) 



Development of the Wall. — In a section of plant tissue under a 

 microscope of moderate power the partition between any two cells 

 appears as a triple structure (Fig. 54, a). In a meristematic tissue, such 

 as that in the root tip or the cambium, the three layers are very thin 

 and semifluid, but all are present: each cell has its own cellulose wall 

 lying against an intervening layer of noncellulosic intercellular substance. 

 The outer wall of an epidermal cell is, of course, single. We shall now 

 recall the development of this condition as described in the foregoing 

 chapter (page 65) and then proceed with an account of the further 

 changes that transform the early walls into the elaborate structures seen 

 in such mature tissues as wood. The successive stages in the entire 

 process are represented diagrammatically in Fig. 51. 



75 



