THE SEEDLING AND YOUNG PLANT. 63 



about six to ten fibers end in a single parenchyma cell, 

 and thus are formed short, tangential rows of wood- 

 parenchyma cells, intercalated, as it were, between the 

 radial rows of other elements (Fig. 12, p). It often hap- 

 pens, moreover, that reticulated and pitted vessels are 

 closely surrounded by wood-parenchyma. 



The secondary medullary rays exist as single radial 

 rows of cells, agreeing in form, etc., with the cells of 

 the primary medullary rays. In contact with one an- 

 other or with wood-parenchyma their walls have simple 

 pits, but they have bordered pits where they abut on 

 tracheids or vessels. In winter these cells are filled 

 with starch. On tangential sections (Fig. 15) it is easy 

 to see how the vertical groups of cells have the same 

 origin as the groups of wood-parenchyma cells the 

 difference being that the cambial cells which are going 

 to be transformed by horizontal divisions, etc., into ver- 

 tical rows of ray parenchyma, undergo repeated tangen- 

 tial longitudinal divisions, and so continued radial rows 

 are formed. The cells of these rays are often much 

 shorter than those of the wood-parenchyma, yet all gra- 

 dations occur. The mother-cells may be very long, 

 evidently corresponding to two, and they may also di- 

 vide in the radial longitudinal plane, and the ray be- 

 come biseriate. 



These secondary rays start (on the transverse section) 

 from the first large vessels, or from younger ones, or they 

 may start from other points. The ray may sometimes 

 cease within the first year's bundle ; but the difficulty 



