514 MacDougal: Symbiosis and Saproph\tism 



outer surface, which may in some instances assume a papillose 

 form, or may be extended to form typical root-hairs. The latter, 

 and all of the epidermal elements, are rich in protoplasm. The 

 outer walls of the epidermis are covered more or less thickly with 

 crystals of calcium. The sub-epidermal layer is hardly differen- 

 tiated, and consists of muriform elements rich in protoplasm, and 

 devoid of reserve material. The cortex is composed of ovoid, 

 cylindrical or globose cells, the outer and inner layers of which 

 are composed of elements slightly smaller than those of the middle 

 region. The middle and inner regions are heavily loaded with 

 starch, especially in the older or basal portions, and the cells of 

 the middle region show a tendency to elongation in the radius of 

 the root. Raphide cells are indifferently distributed throughout 

 the cortical tissues, but perhaps more abundantly in the outer 

 layers. Two types of roots which do not differ greatly in outward 

 aspect may be distinguished ; a fibrous form, and a fleshy form 

 about twice the diameter of the first. External to the endodermis 

 the two are similar except in the amount of development of the 

 cortex. In the development of the thicker storage organs, the 

 central parenchyma becomes slightly lignified and sclerotized, the 

 xylem bundles increase from six to seven or eight, the pericycle 

 shows two or three layers underneath the endodermis, and this 

 sheath is composed of slightly thickened and pitted elements with 

 no special passage cells. The xylem and the endodermis become 

 lignified (Plate 369, Fig. 2). In the development of the fibrous 

 roots the xylem undergoes such centripetal development, and 

 lateral fusion that it changes from a hexarch to a tetrach. The 

 heavily lignified xylem and the weakly developed central paren- 

 chyma form a four-rayed star, with the phloem lodged in the 

 sinuses (Plate 369, Fig. 2). The endodermis is heavily thickened 

 in the regions external to the phloem regions and consists of thin- 

 walled passage cells opposite the xylem. The xylem shows great 

 degeneration in both types of roots, and consists of scalariform 

 vessels and elongated tracheids with transverse pits. The phloem 

 exhibits no ereat deviation from the normal, and consists of narrow 

 sieve tubes, companion cells and some parenchyma. A layer of 

 elongated elements containing protoplasm and apparently adapted 

 to the conduction of proteids lies immediately outside the phloem 



