114 ANATOMY. OF STEM, ♦ROOT AND LEAF 



bands running radially between the bundles and connecting the 

 cortex with the medulla are the medullary rays (iti). 



The vascular bundles, together with the medullary rays and 

 pith, form a cylindrical mass of tissues known as the vascular 

 cylinder or side, which extends continuously throughout the 

 plant from the tip of the stem to the growing-point of the 

 root. 



(i) The epidermis is usually one cell thick and acts as a pro- 

 tective coat for the plant, preventing the latter from too rapid 

 loss of water and also defending the deUcate internal cells of the 

 plant against mechanical injuries due to rain, hail, frost and 

 insect attacks. 



The cells are tubular flattened cells fitting quite closely to- 

 gether, except where the openings named stomata occur : as 

 the latter are more abundant in the epidermis of a leaf, their 

 structure is deferred to page 145. Usually the outer cell-wall 

 of each epidermal cell is much thicker than the lateral and 

 inner walls, and is differentiated into two or three layers, the 

 outermost layer in contact with the atmosphere being spoken of 

 as the cuticle. The cuticle is composed of a substance known as 

 cuiose, which is very impervious to water, and a remarkably stable 

 body capable of resisting the action of various solvents which 

 dissolve ordinary cellulose. 



On the cuticle of the stems and leaves of cabbages, swedes, and 

 many varieties of cereal and other grasses, as well as on grapes 

 and plums, an ash-coloured bloom is seen. It is an excreted 

 product of the epidermal cells, and consists of minute round, 

 rod-like or scaly particles of wax. Surfaces of the different parts 

 of plants covered with this bloom lose less water than those 

 from which the substance has been removed by rubbing. 



This waxy layer appears also to act as a partial protection 

 against the attacks of fungi and insects. 



The cells of the epidermis contain the usual cell-contents 

 with the exception of chloroplasts which are generally missing ; 



