MORPHOLOGY OF HIGHER PLANTS. 309 



THE INNER STRUCTURE OF THE ROOT. 



Primary Structure. If we make a transverse section of the 

 young portion of a root (Vascular Cryptogam, Gymnosperm, or 

 Phenogam), we notice the following tissues (Figs. 169-174). 

 The outermost tissue is EPIDERMIS (E), it being generally thin- 

 walled and destitute of cuticle ; it is, as a rule, hairy, and these 

 hairs, which are relatively long, but always unicellular, are known 

 as ROOT-HAIRS (Figs. 161, 1 68) ; they ramify but very seldom. 

 Inside the epidermis there is frequently present a HYPODERMIS 



FIG. 169. Radial vascular bundle in root of Allium ascalonicum, showing a large 

 central trachea from which radiate five small groups of tracheae and between which are the 

 groups of leptome or sieve; p, layer of pericambium or pericycle; d, transition cells or pas- 

 sage cells in the endodermal layer, and which permit the easy transfer of substances between 

 the cortical parenchyma and the tracheae of the stele. After Haberlandt. 



(sometimes referred to as an EXODERMIS) composed of a single 

 layer of cells or, at the most, of but several layers, the cells of 

 which differ in shape and size from those of the epidermis and the 

 adjoining cortical parenchyma. The hypodermis takes the place of 

 the epidermis when the latter is worn off, except in the few cases 

 where hypodermal cork becomes developed, as in Cephalanthus, 

 Solidago, and in the Bignoniaceae. 



The root bark is composed of parenchymatous cells, being 



