706 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS 



which is taken in a direction at right angles to that of the medul- 

 lary rays, and therefore cuts them across, we see that each of the 



FIG. 547. Portion of vertical section of the 

 same wood, taken in a radial direction, 

 showing the trachei'ds with ' bordered 

 pits,' without ducts, crossed by the medul- 

 lary rays, a, a. 



FIG. 548. Portion of vertical 

 section of the same wood, 

 taken in a tangential direc- 

 tion, so as to cut across the 

 medullary rays. 



plates thus formed has a very limited depth from above downwards, 

 and is composed of no more than one thickness of cells in the 

 horizontal direction. A section of the stem of mahogany taken in 

 the same direction as the last (fig. 549) 

 gives a very good view of the cut ends of 

 the medullary rays as they pass between 

 the prosenchymatous cells ; and they are 

 seen to be here of somewhat greater thick- 

 ness, being composed of tw r o or three rows 

 of cells, arranged side by side. 



In another fossil wood, whose transverse 

 section is shown in fig. 550, and its tan- 

 gential section in fig. 551, the medullary 

 rays are seen to occupy a much larger part 

 of the substance of the stem, being shown 

 in the transverse section as broad bands 

 (a a, a a) intervening between the closely 

 set prosenchymatous cells, among which 

 some large ducts are scattered ; whilst in 

 the tangential section they are observed 

 to be not only deeper than the preceding 

 urnvi v M^V'MU^ i. fr m above downwards, but also to have 



,. a much greater thickness. This section 

 FIG. 549. Vertical section of . . & ., , 



mahogany. a l so gives an excellent vie\v ot the ducts, 



b 6, b 5, which are here plainly seen to be 



formed by the coalescence of large cylindrical cells lying end to end. 

 In another fossil wood in the Author's possession the medullary rays 



