144 



PART II. ANATOMY AND HISTOLOGY. 



[35 



comes crusted and flattened by the development of the more inter- 

 nal layers subsequently formed. 



The structure of the secondary conjunctive tissue (medullary 

 rays). The cambium-ring not only adds to the existing primary 

 medullary rays, but gives rise to new (secondary, tertiary} rays in 

 the successive years of growth (see Fig. 112), amongst the vascular 

 tissue. 



The cells of the medullary rays are typically parenchymatous, 

 somewhat brick-shaped, with their long axes along radii from the 



FIG. 114. Radial longitudinal section of the wood of the stem of a Pine, along the length 

 of a medullary ray q p q, consisting of six horizontal rows of cells, one above the other : 

 t tracheids with bordered pits ; the tracheids h with smaller bordered pits are the autumn- 

 wood of one year, those to the right with larger pits constitute the spring-wood of the next 

 year ; q tracheidal elements of the medullary ray ; p true cells of the ray : where the cells 

 of the medullary ray abut on the tracheids the pits are simple and large ( x 300). 



centre to the periphery of the member (Fig. 112); their more or 

 Irss thickened walls are lignified, and they have protoplasmic eon- 

 tents. Occasionally, however, some of the cells of a ray lose their 

 protoplasmic contents and constitute tracheids (e.g. Abietinese, 

 Fig. 114 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 p_assing_radially 



