72 



BOTANY OF THE LIVING PLANT 



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. 53. 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 r<?c^A^zV5. (Figs. 53,54.) 

 Below the piliferous layer comes 

 the bulky tissue of the cortex, of 

 w^hich 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. 54). This prevents leakage of 

 water outwards, and throw^s the control of its passage upon the 

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

 exodermis is thus a living physiological barrier. Much the same is 

 the case with the innermost layer, which is called the endodermis. Its 

 cells also are in close lateral relation one to another, and the radial 

 walls have also a corky band which serves a similar purpose. The 

 cortex thus composed forms the larger part of the area of transverse 

 section, and it may be regarded as a water-reser.voir round the stele, 

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

 of living cells. 



tiG. 54- 

 External tissues of the root of Ruscus, showing 

 each cell of the piliferous layer grown out into a 

 root-hair. ^a^ = exodermis. ( x loo.) 



