THE VASCULAR TISSUES. 71 



In transverse sections of such a stem as Finns (iirst or second 

 year's growth) these rays may be seen as single lines of oval 

 elements which show up well by treating the section with iodine 

 or Schulze's solutions, the reaction being due to the darkening 

 of the starch-granules in the component cells of the ray by the 

 reagent (see Fig. 53). 



To examine the origin and relations of the true medullary 

 rays it is necessary to make longitudinal sections in directions 

 parallel to, and at right angles to, radii of the stem — viz., radial 

 longitudinal and tangential longitudinal sections. 



In radial sections, which include the cambium, each ray is 

 seen to originate from certain cells of the cambial layer. 

 Usually more than one cambial cell is active, and often as many 

 as five or six may be the forerunners of the same number of 

 radial lines of ray-cells. As observed in such a section, the shape 

 of each component cell of the ray is rectangular, and numerous 

 simple pits may be detected in the rather thick cell-walls. The 

 contents of each cell consist of protoplasm and starch-grannies 

 (see Fig. 52). 



In tangential sections of the xylem, each ray appears as a 

 spindle-shaped perpendicular line of cells, ranged one above 

 the other (see Fig. 54). The number of cells in the tier 

 depends, of course, upon the relative position of the plane of 

 section. 



In function the true medullary rays act as reservoirs of carbo- 

 hydrate food-material, being supplemented to a certain extent by 

 the wood-parenchyma. In the early spring, when the sap is 

 beginning its upward movement in the xylem, the starch in the 

 ray-cells is converted by the agency of the enzyme diastase into 

 dextrins and sugar, of great value to the cambium and young 

 xylem and phloem elements, before the sap has started to be 

 elaborated in the leaves in quantity sufficient for the needs of 

 the plant. 



2fote. — The true medullary rays are best studied in the stems of Pinus, 

 or the Lime. In the latter, the rays are, In transverse section, seen to be 

 very broad towards the cortical ends and narrow towards the pith, the 

 increased breadth at the outer extremity being due to the occurrence of 

 radial, as well as tangential, divisions in the component-cells of the ray. 

 Sections of stems should be treated with Schulze's solution or iodine to 

 show up the ra3'3. 



