

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 



st> st 



jpfv 



FlG. 43. LONGITUDINAL SECTION OF 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 ; ph, 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 



it is most easily seen in thickened 



cells, it is probably not confined to 



LAMELLA - ( After Sachs -> 



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



