[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES 



301 



Fig. 120. Section of stem of dodder above and a cross section of petiole of host 

 plant below. A haustorium from the dodder has grown through the cortex to a 

 vascular bundle in the petiole. 



cross section of a woody twig, the same kind of tissues in the same order 

 of arrangement may be seen, except that no pith will be found in the 

 root. From the center of this woody root outward therefore, there are, in 

 order of occurrence: xylem, vascular cambium, phloem, pericycle, en- 

 dodermis, cortical parenchyma, cork cambium, cork, and epidermis if it 

 has not already become broken and sloughed off (Fig. 121). In most 

 of the roots that have been studied the first cork cambium originated in 

 the pericycle and the cortex was soon sloughed off. 



The larger roots of a tree, like the larger stems above ground, consist 

 only of the vascular cylinder (stele) and the cork that develops peri- 

 odically from temporary cork cambiums derived from pericvcle or from 

 phloem parenchyma. Obviously, water and mineral salts do not enter 

 roots where they are enclosed by a cylinder of cork. They enter only 

 the youngest zone of tree roots that have not become enclosed in cork. 



The growing root tips of manv plants are sufficiently transparent so 

 that when they are examined with a hand lens or low-power microscope, 

 one may easily recognize the terminal root cap, the central vascular 

 cylinder surrounded by a cortical cylinder, and the numerous root hairs 

 which protrude from the epidermis a short distance back of the apex 

 ( Fig. 122 ) . Root hairs are not present on the roots of all species of plants, 

 and their abundance on others varies in different environments. 



