148 



GENERAL STRUCTURE OF ROOT AND STEM 



rays now appear as narrow lines. That is, we now see the edges of the 

 blades whose sides were before seen. Passing outward beyond the 

 last of the annual rings, which successively exhibit a greater number of 

 wood-bundles and medullary rays, we reach the cambium-ring. Outside 

 of this we find the phloem or bast bundles sejjarated by medullary rays 

 continuous with those of the wood cylinder, and still outside of this the 

 periderm. 



Fig. 423. Diagram illustrating section of woody portion of dicotyledonous stem: a, edges of medul- 

 lary rays as seen in transverse section; 6, sides of same as seen in radial section; c, ends as they would 

 appear in tangential section. 



Appearance of the Tangential Section. — The appearance of a tangen- 

 tial section will depend, of course, upon the tissues through which it 

 passes. If it cuts the medullary rays these will appear neither as the 

 broad sides, as at h, nor the edges of blades, as at a, but as transverse 

 sections of them, as at c. If the ray consists of but one row of cells in 

 width, then such a row will be exhibited upon the tangential section, its 

 vertical height varying from a very few to quite a large number of cells. 

 If, upon the other hand, it possess a lateral breadth of several thick- 

 nesses of cells, of 5 in our figure, this condition will exist only at its 

 middle portion. At its upper and lower limits it will ordinarily be 

 reduced to the thickness of a single cell, so that the tangential aspect 

 of a medullary ray is almost always that of an ellipse, broad or narrow, 

 according to the numl)er of rows of cells of which it consists, in contrast 

 with the extent of its upward and downward extension. 



