Knowledge of Rachiopteris cylindrica , Will. 549 
The vascular supply of each root is connected with a group of rather 
small tracheides occurring at the periphery of the stem xylem 1 ; the details 
of root-formation are, however, difficult to observe. 
The passage of the roots through the cortex is variable ; it may be 
horizontal (PI. XXVI, Fig. 3), or more or less vertical (PI. XXVI, Fig. 8). 
ii. Anatomy and Histology .—The roots of a and ft types are similar in 
structure; they vary considerably in size, an average diameter being o -6 mm.; 
and they possess a diarch xylem-plate consisting of a few tracheides (PI. 
XXVII, Fig. 3), all of which appear to be typically scalariform, although here 
Text-fig. 12. A portion of a well-preserved root, showing in transverse section the phloem- 
cells (ph.), and the endodermis ( e .), the darkened cell-contents of which have here and there a pitted 
appearance; px., protoxylem; pc ., pericycle. x 400. (From slide K21W, University College, 
London.) 
and there are indications of spiral thickening in the case of the protoxylem 
elements. The phloem is usually not well preserved ; in a few cases, 
however, it may be recognized as a group of small thin-walled cells on 
either side of the xylem plate, alternating with its poles (Text-fig. 12). 
There appears to be a little conjunctive parenchyma, and occasionally 
a pericycle may be observed underlying the endodermis (Text-fig. 12). 
The cells of the pericycle are rather irregularly arranged, sometimes 
alternating with those of the endodermis, at other times being opposite to 
them. The endodermis is strongly marked, particularly in the older roots 
(PI. XXVII, Fig. 3); it has the same characters as that of the stems and 
petioles, the pitted appearance often being very pronounced (PI. XXVII, 
Fig. 3 ; Text-figs. 12 and 13) in both transverse and longitudinal sections 
1 Cf. Scott (’ 08 ), p. 329; and Seward (’ 10 ), p. 438. 
