236 PRACTICAL BOTANY 



and inwards towards the pith : they are the medullary 

 rays. (Compare Fig. 9, A, row 2.) Some of them may 

 be traced the whole way to the cortex and to the pith 

 (primary medullary rays), others only part of that 

 distance (secondary medullary rays). 



Note that the cells of the medullary rays at the 

 cambium-zone are less elongated radially than in the 

 xylern or phloem, the cambium being the formative 

 region of the rays as well as of the other tissues. 



The mature cells of the ray usually have cellulose 

 walls (blue with chlor-zinc-iodine), and granular proto- 

 plasmic contents with nucleus. In fact the cells of the 

 medullary rays usually retain their cell-nature. 



Follow the radial rows of cambium-cells outwards, 

 and note the gradual transition to the permanent tissues 

 of the secondary phloem, the constituents of which 

 are also arranged in radial rows, and have cellulose 

 walls (blue with chlor-zinc-iodine). The ring of second- 

 ary phloem is cut up into rectangular areas by the 

 medullary rays, which are easily recognized as above 

 directed. Observe that the tissues filling these areas 

 are of three sorts 



i. Elements with cellulose walls, and no very distinct 

 contents : they are radially compressed : these are the 

 sieve-tubes, which compose the greater part of the 

 phloem. The walls are differentiated into layers, and 

 have bright globules attached to them. 



ii. Here and there the radial rows of sieve-tubes are 

 broken by single large cells of the bast-parenchyma, 

 which resemble in their characters those of the 

 medullary rays. 



iii. Towards the periphery of the phloem are elements 



