THE SKELETON OF THE PLANT 



45 



In cell-walls which have undergone considerable thick- 

 ening the membrane shows a marked differentiation. The 

 centre of the wall is found to possess a chemical composi- 

 tion very unlike that of the thickening layers. It marks 



T IV' p W /> 



St St 



FIG. 43. LONGITUDINAL SECTION OP VASCULAR BUNDLE OF SUNFLOWER 

 STEM. (After Prantl.) 



p', pith ; s, s', spiral vessels ; w', w, wood-cells ; p, p, pitted vessels ; 

 c, cambium; st, st, sieve-tubes;^/?, fibres; n, bundle sheath; 

 c, cortex. 



off the limits of the cells, occupying the position of the 

 original thin membrane and looking as if it were the 

 basis on which the thickening 

 layers have been deposited. When 

 a piece of tissue is warmed gently 

 with a mixture of potassic chlorate 

 and strong nitric acid, this layer 

 dissolves and the cells become sepa- 

 rated from each other. It has by 

 certain writers been termed the 

 intercellular substance and by others 

 the middle lamella (fig. 44). Though 



.",.-, , FIG. 44. THICKENED WOOD- 



it IS most easily Seen in thickened CELLS, SHOWING MIDDLE 



cells, it is probably not confined to LiM 



them, but exists in all cell-membranes, even when they are 



