474 



SECONDARY CHANGES. 



follow the growth in thickness chiefly by means of radial extension, so that, as seen in 

 transverse section, the small-celled ring appears interrupted at the xylem-plates by 

 rows of larger cells, e. g. Cucurbita, Urtica (Figs. 203, 204). 



The production of tissue by the cambial ring or its segments, when once 

 originated, maintains the same course as that described in the case of the stem. 

 Any special differences in the succession of the divisions are, at the present time at 

 least, unknown. The mass of tissue given off internally is to be termed the wood, 

 the peripheral mass, the bast. Both are divided into wood and bast bundles, which 

 correspond to one another in the same way as in the stem, and may be compre- 

 hended under the name strands, and into radial bands composed of non-equivalent 

 elements, which alternate with the strands, and in the root no doubt always consist 

 of parenchyma ; these are the medullary rays. 



The original large medullary rays are obviously excluded in the case of the 

 root. As regards the arrangement of those rays which are present, and the dis- 



y- 



Fiff. 203. 



Fig. 204. 



Fig. Z03. — Urtica dioica; transverse section of a small subsidiary root from the rhizome (So), g original diarcli xylem- 

 plate ; crossing it are two secondary bundles of wood, separated by broad bands of parenchyma (medullary rays) ; outside each, 

 at J, is the bundle of bast belonging to it. Secondary cortex is present all round, with numerous scattered sclerenchymatous 

 fibres, indicated as dark spots ; the whole bounded externally by periderm, c layer of cambium or young secondary growth. 

 The primary cortex hsis been cast off, 



FIG. Z04.— Cucurbita Pepo ; transverse section of the main root of a young plant {40). £■ xylem body of the axial bundle, 

 its four rays united in the middle by a large pitted vessel ; four secondary bundles of wood alternate with them ; s the original 

 and secondary phloem-bundles. The primary cortex of the root has been Replaced by periderm and cast off. 



position of the strands which is determined by them, two main* types are to be 

 distinguished, though these are not sharply contrasted in all cases. 



(i) A main medullary ray (usually very broad) appears opposite the angle of 

 each original xylem-plate, the same number of main strands alternating with the 

 main medullary rays, e.g. Centranthus, adventitious roots of Tropseolum, Urtica 

 dioica (Fig. 203) among diarch roots; the main root of Cucurbita (Fig. 204), 

 Phaseolus, Convolvulus tricolor, and many others among the tetrarch forms ; adventi- 

 tious roots of Cereus grandiflorus, Clusia, Cucurbita, and Artanthe among the poly- 

 arch root-bundles. Comp. van Tieghem, /. c. 



(2) The whole periphery of the primary tiundle acquires fascicular elements, 



