STRUCTURE OF THE ROOT 



83 



section will be circular. The general arrangement of the tissues, 

 exposed in a section cut about two or three inches behind the apex of 

 any ordinary root, is more regular than that usual in stems. It is 

 shown diagrammatically for a thin root, such as that of a Pea seedling, 

 in Fig. 56. There is a superficial covering which may be held as 

 corresponding to the epidermis. Within it lies the cortex, and 

 centrally the stele. But here the cortex is relatively bulky, while 

 the stelar column is much more contracted than is usual in stems. 

 This disposition of the tissues is 

 typical for roots at large. 



The superficial layer consists of 

 an unbroken series of thin-walled 

 cells, without any cuticle upon 

 their outer walls. It is called 

 the piliferous layer because many 

 of its cells are extended out- 

 wards as root-hairs (Figs. 57, 66). 

 Below the piliferous layer comes 

 the bulky tissue of the cortex, of 

 which the outermost and the 

 innermost layers are sharply de- 

 fined, while the massive band of 

 tissue between them consists in 

 the young root of a featureless, 

 thin-walled parenchyma with in- 

 tercellular spaces. The outer- 

 most layer, lying directly below 

 the piliferous layer, and with its 

 cells alternating with these, is called the exodermis. Its cells fit closely 

 together, and they show a sharply defined corky band upon their radial 

 walls, which often extends with age to the other walls (Fig. 57). This 

 throws the control of the passage of water upon the protoplasts 

 of the living cells so long as the root is young. The exodermis is 

 thus a living physiological barrier, as is also the jnnermost layer, 

 which is called the endodermis. Its cells also are in close lateral 

 relation one to another, while the radial walls have a corky 

 band which serves a similar purpose (Figs. 58, 59). The cortex 

 thus composed forms the larger part of the area of transverse section, 

 and it may be regarded as a water-reservoir round the stele, con- 

 trolled both on its outer and its inner surfaces by the protoplasm 

 of living cells. 



Fig. 57. 



External tissues of the root of Ruscus, showing 

 each cell of the piliferous layer grown out into a 

 root-hair. ex= exodermis. The section shown was 

 probably taken above the absorbing region. 

 ( x 100.) 



