122 MEDULLARY RAYS 



The wood-parenchyma cells of the secondary xylem resemble 

 those of the primary xylem, in form and in the possession of a 

 living protoplast ; the walls are, however, commonly thicker 

 and often lignified, the horizontal ones showing prominent pitting 

 (Fig. 60, p). Wood-parenchyma cells may be generally dis- 

 tributed throughout the wood (e.g. Birch, Beech), or more especi- 

 ally confined to the autumn-formed wood and the immediate 

 vicinity of the vessels, sometimes completely ensheathing them 

 (e.g. Ash). The parenchyma in contact with the vessels bears 

 simple pits corresponding in position to pits of the vessel wall, 

 which latter are here either simple (e.g. Oak) or bordered (one- 

 sided bordered pits), like the remaining pits of the vessel wall. 



The structure of the medullary rays can only be fully appre- 

 ciated by a study of their appearance in transverse, radial, and 

 tangential, longitudinal sections. In the transverse section the 

 cells, except for the fact that their long axis is placed radially 

 and not vertically, are very similar to those of the wood-paren- 

 chyma, though not uncommonly having somewhat thinner walls. 

 The latter bear simple pits which are often particularly numerous 

 on the tangential walls (Fig. 60, B; Fig. 62, m.). The simple 

 pits of the radial walls and the bordered pits of the vessels com- 

 bine to form one-sided bordered pits like those described above. 



Attention has already been drawn to the variable width of 

 the rays. In general the secondary medullary rays are narrow 

 and often only one cell wide ; the primary ones in most trees 

 do not greatly exceed the secondary in width, but in many herbs 

 the contrast is extremely marked. Each ray is a plate of cells 

 of which the full vertical extent is seen in tangential longitudinal 

 sections (Fig. 60, C, m.r.). The secondary medullary rays are 

 very limited in this direction, rarely exceeding ten or twelve 

 cells in height, whilst the primary rays usually extend through 

 an entire internode. Each ray, as seen in tangential section, is 

 somewhat spindle-shaped as a result of the tapering of the cells 

 at the upper and lower margins (Fig. 60, C, m.r.). 



In radial longitudinal sections the secondary rays are cut 

 parallel to their flat faces, and appear as so many narrow paren- 

 chymatous strips passing at right- angles across the longitudinal 

 grain of the wood (Fig. 60, B). The detailed structure of the 

 ray here somewhat resembles that of a brick wall, the alternation 



