ROOTS. 89 



cells on the outside, the Epidermis, which may or may not 

 be supplied with hairs. The walls are usually slightly 

 thickened and stained dark brown, owing to the process of 

 suberization. Beneath the layer of cells constituting 

 the epidermis are to be found several layers of irregular 

 parenchyma tic cells, the Cortex, the outer cells of which 

 abut on the epidermis ; in some cases they are thickened 

 and form a special layer or layers, the Hypodermis. The 

 parenchyma of the cortex is frequently filled with starch 

 and often contains cells which contain crystal sacs. 

 Toward the periphery the cells of the cortical portion are 

 more or less regularly arranged, but toward the centre 

 they become more irregular. The innermost layer of 

 the cortical portion is often differentiated into a dis- 

 tinct layer of regularly arranged cells. It is then termed 

 the Endodermis, and serves a number of purposes. 

 Its cells are often characteristic and diagnostic, very 

 frequently the w^alls become peculiarly thickened and 

 marked and starch grains are found in their cavities. 



The central cylinder (Intrastelar Tissue) varies widely 

 in the different roots. It is in this portion of the roots 

 that the differences between primary and secondary 

 structures are most prominent. The layer of cells just 

 beneath the endodermis is usually distinct from the rest 

 of the central cylinder. It is termed the Pericycle or 

 the Pericambium, and consists of thin-walled parenchy- 

 matic cells. Inside arc the vascular tissues or rudimen- 

 tary fibro- vascular bundles. These, in most of the roots 

 studied by the pharmacognocist, are arranged in a radial 

 manner, the conjunctive tissue being about them, and in 

 the center forming the pith. In some cases the fibrous 

 portion of the bundles occupies the center (Veratrttm 

 viride). The number of fibro- vascular bundles varies 

 greatly. Many roots are monostelar, but individual 

 variations are constant. 



Secondary Structures. — In cryptogams and most mono- 



