142 PRACTICAL BOTANY. 



others only part of that distance (secondary medullary 

 rays). 



Note that the cells of the medullary rays at the cam- 

 bium zone are less elongated radially than in the xylem 

 or phloem ; the cambium being the formative point of 

 these tissues. 



The mature cells of the ray usually have cellulose 

 Avails (blue with Schulze's solution), 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 Schulze's solution). The ring of secondary 

 phloem is cut up into rectangular areas by the 

 Medullary rays, which are easily recognised 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 (yellow with 

 Schulze's solution). 



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 similar in form to the sieve-tubes, whose cell 

 contents are brown, and contain crystals. 



