STRUCTURE OF RADIAL BUNDLES. 



3^3 



tracheides, which forth several rows, are of a different structure, similar to that 

 described in the case of the stem and leaf at p. 346, and are all of approximately 

 equal and relatively small width. Triarch and tetrarch bundles sometimes occur in 

 thick roots of species, which are usually diarch ; triarch-bundles have been observed 

 in Pilularia, Equisetum, Botrychium, Blechnum brasiliense, and Cyathea meduUaris, 

 tetrarch in Equisetum, the Blechnum above mentioned, and Cyathea. In the species 

 of Trichomanes^ investigated, triarch to octarch bundles usually occur, diarch bundles 

 being rare, while, on the other hand, the latter are characteristic of the roots of Hymeno- 

 phyllum. On the monarch bundles of some species of Trichomanes, see below. 



FIG. 169. — Adiantum Moritzianmn (225), Old root, cross-section. A — A hairs of the epidermis cut through, ti endodermis 

 /c pericainbium, /r primitive tracheides of the diarch xylem aJtemating with two phloem groups. 



The xylem-plates are in most cases united in the middle, in the thinner bundles 

 often by means of a very large vessel (e. g. Equisetum), or of a row consisting of 

 two large vessels, crossing the diametral pair of plates at right angles (Fig. 169). 

 In other respects various subordinate differences of form occur, e. g. a regularly 

 elliptical cross-section of the diametrally united plate in Osmunda, Todea, &c., &c. 



The arrangement of the phloem-groups corresponds to the general plan of 



' Mettenius, Hymenophyllaceen, /. c. p, 420. — Russow, /; c. p. 95. 



