CH. IV] CENTRAL CYLINDER. 55 



it differs from it strikingly in detail. The resemblances are, 

 however, of a more fundamental nature than the points of 

 difference. The chief part of the section is taken up by 

 a cylinder, the central cylinder, which corresponds to the 

 region bearing the same name in the root, with which it 

 is indeed continuous. In the diagram, fig. 22, the cy- 

 linder is marked out by the double line e. Outside the 

 central cylinder is a region known as the cortex : the cortex 

 is covered by a single layer of cells ep, forming a special 

 tissue known as the epidermis. This tissue is of great 

 importance both morphologically and from the point 

 of view of function, and will be considered in detail 

 in a later chapter. The most internal layer of the cortex 

 is the line e, fig. 22, already referred to, which bears a 

 name identical with the corresponding layer in the root, 

 viz. endodermis. In the stem it generally pursues a 

 wavy course, as in the figure, and may be easily recognized 

 by the presence of starch grains in its cells. In the 

 centre of the central cylinder is a large mass of pith, p : 

 in the growing condition the pith is juicy, soft and greenish 

 in colour ; but after a time, long before the whole plant 

 dies, the pith changes its character : its cells die, that is 

 to say, the protoplasm inside them dies ; they no longer 

 contain cell-sap but become filled with air. It is now 

 no longer a green and sappy, but a dry, white, spongy and 

 very light substance, like the pith of a woody elder 

 branch, or like the pith used in the laboratory in cutting 

 sections. 



Surrounding the pith is a broken ring, made of a 

 series of dots; this ring feels hard and woody to the 



