THE WORK OF STEMS 233 



In the wood of the Oak (Fig. 135) we find the same 

 elements as in the Squash, namely, ducts (rf), wood 

 parenchyma (wp) , and long, pointed tracheids (tr) , 

 dovetailed together at the ends. Most of the tracheids 

 in the Oak are pitted; tracheids with spiral or ring- 

 shaped thickenings occur only in the innermost wood 

 next to the pith. The wood parenchyma cells are 

 usually distinguished by being filled with starch, so 

 that by placing the section in iodine they stand out 

 prominently; at the same time the medullary rays 

 (mr), or silver grain of the wood, stand out unmis- 

 takably by reason of their starch -content. Whether 

 we look at the cross or the longitudinal section, the 

 starch, by its dark coloration, forms a distinct pattern, 

 which shows very clearly the course it travels from the 

 outer green -celled tissue of the rind, inward through 

 the narrow, ribbon-like medullary rays, and from 

 them into the intersecting bands of wood parenchyma 

 in which it travels freely up and down the stem. 

 The form of the medullary rays (mr) is easily under- 

 stood from the figure. They consist of cells elongated 

 in the direction of the radius of the stem, and run from 

 the rind (or cortex) through the bast into the wood, 

 many of them reaching clear to the pith, to which 

 they convey large quantities of starch. In the longi- 

 tudinal section the cells of medullary rays are shown in 

 surface view (i. e., not -cut open), consequently we see 

 the pits which occur in their walls; it is principally 



