72 



OAK. 



[CH. V 



walls are thickened, lignified and pitted, and enclose 

 living protoplasm "and a good deal of starch. The bigger 

 medullary rays are several cells in width, while the smaller 

 ones are but one cell wide, but this of course does not 

 show in longitudinal section. In either case the ray 

 is a plate of cellular tissue with its edges pointing 

 upwards and downwards. The medullary rays are of 

 various depths (i.e. in the direction of the axis of the 

 branch), the primary rays being the deepest. The exact 

 form of the ray can best be seen in a tangential section. 

 In fig. 31 the line T represents such a section, and it is 

 clear that, since the medullary rays run like radii from 

 the circumference towards the centre, they must be cut 

 by T. Thus a tangential section of the branch gives 

 approximately transverse sections of the medullary rays. 

 The rays represented in fig. 33 are one cell in thickness 



FIG. 33. 



LONGITUDINAL TANGENTIAL SECTION OF THE WOOD OF THE OAK, 



showing dotted vessels and tracheids among which are 



the medullary rays. 



