412 



STRUCTURE OF DICOTYLEDONOUS STEM. 



Fig. 218. 



and crossing in a horizontal direction the Woody Fibres which lie 

 parallel to one another vertically. And in the Tangential section 

 (Fig. 217), which passes a direction at right angles to that of the 

 Medullary rays, and therefore cuts them across, we see that each 

 of the plates thus formed has a very limited depth from above 

 downwards, and is composed of no more than one thickness of 

 horizontal Cells. — A section of the Stem of Mahogany taken in 

 the same direction as the last (Fig. 218), gives a very good view 

 of the cut ends of the Medullary Rays, as they pass between the 



Woody Fibres ; and they are seen 

 to be here of somewhat greater 

 thickness, being composed of two 

 or three rows of Cells, arranged 

 side by side. — In another Fossil 

 Wood, whose Transverse section 

 is shown in Plate xiii., fig. 1, and 

 its Tangential section in fig. 2, 

 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) inter- 

 vening between the closely-set 

 Woody Fibres, among which some 

 large Ducts are scattered ; whilst 

 in the Tangential, they are ob- 

 served to be not only deeper than 

 the preceding from above down- 

 wards, but also to have a much 

 greater thickness. This section 

 also gives an excellent view of the 

 Ducts b b, b b, which are here 

 plainly seen to be formed by the 

 coalescence of large cylindrical 

 Cells, lying end-to-end. — In ano- 

 ther Fossil Wood in the Author's 

 possession, the Medullary Rays 

 constitute a still larger proportion of the Stem ; for in the Trans- 

 verse section (Plate xiii., fig. 3) they are seen as very broad bands 

 (b, b), alternating with plates of Woody structure (a, a), whose 

 thickness is often less than their own ; whilst in the Tangential 

 section (fig. 4) the cut extremities of the Medullary Rays occupy 

 a very large part of the area, having apparently determined the 

 sinuous course of the Woody Fibres ; instead of looking (as in 

 Fig. 217) as if they had forced their way between the Woody Fibres, 

 which there hold a nearly straight and parallel course on either 

 side of them. The purpose of the Medullary Rays appears to be 

 to maintain a connection between the external and the internal 



Vertical section of Mahogany. 



