35.] CHAPTER II. THE TISSUES. 201 



centre to the periphery of the member (Fig. 149) ; their more or 

 less thickened walls are lignified (p. 132), and they have proto- 

 plasmic contents. Occasionally, however, some of the cells of a 

 ray lose their protoplasmic contents and constitute tracheids (e.g. 

 Abietinege, Fig. 149 q q) ; in some few cases the ray consists of 

 long fibrous cells, in place of parenchyma (e.g. shrubby Begonias). 

 The medullary ray is, then, a strand of cells passing radially 

 among the longitudinally arranged tissues of the wood and of the 

 bast (Fig. 149). Its size varies, even in the' same member, both 

 as regards its vertical (height) and its' lateral (breadth) dimen- 

 sions. With regard to the former, the ray may consist of only a 

 single row of cells (as in Abietinese; Quercus, Fagus) ; the limits 

 may be generally stated at 1-12 rows of cells, though in some 

 cases they are considerably larger than this* when they include 

 resin-ducts (e.g. Abietineae) or other forms- of secretory tissue. In 

 any case, the secondary medullary 

 rays, unlike the primary, do not ex- 

 tend throughout the whole length of 

 an internode. The breadth of the 

 secondary medullary rays is never 

 nearly so great as their height : as- 

 seen in tangential longitudinal sec- 

 tion, they are narrow above and below FlG - ^.-Diagrammatic repre- 



" . . sentation of the course of the 



and broader in the middle ; it IS Only medullary rays in 1 a segment cut 



in the middle that they ever consist out of tne wood of a tree-trunk. 



. Q' Horizontal surface; E Radial 



Ot more than one rOW Ot Cells in surfate . T Tangential (external) 



breadth, the upper and lower margins surface of the wood ; the shaded 



... -\ i -inT -L. portions m are the medullary rays. 



consisting ot a single row only. With 



regard to their radial extent, it is only the primary medullary 

 rays which extend from pith to' pericycle ; the subsequently formed 

 rays (secondary,- tertiary, etc.) extend between the wood and the 

 bast of the year in which they were formed. 



As instances of especially large secondary medullary rays should 

 be mentioned those formed in roots (see Fig. 146, p. 194) where the 

 cambium forms only conjunctive tissue opposite' the prima.ry 

 xylem-bundles. 



In some few stems the formation of secondary conjunctive tissue 

 is especially connected with the primary medullary rays (e.g. 

 Cucurbitaceaa, Menispermaceae,- woody Piperacese,- Aristolochia, 

 Casuarina, Atragene, Begonia, Berberis, etc., see p. 193). In these 

 plants the formation of secondary vascular tissue is confined to the 



