PINE-STEM 137 



Observe, here and there, radial rows of which the cells are 

 more elongated in a radial direction than the rest : these may 

 be traced outwards towards the cortex and inwards towards the 

 pith : they are the medullary rays. (Compare Fig. 7, 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). 



The mature cells of the ray usually have cellulose walls (blue 

 with chlor-zinc-iodine), and granular protoplasmic 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 secondary 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 con- 

 tents : 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 similar 

 in form to the sieve-tubes, whose cell-contents are brown, and 

 contain crystals. 



Note on passing to the periphery of tLa phloem an in- 

 creasing irregularity of form of the tissues, due to dis- 

 tortion, caused by pressure from without by the cortical 

 tissue upon the vascular system as it increases in bulk by 

 secondary thickening. 



Follow the radial rows of cambium-cells inwards, i.e. towards 

 the centre of the stem. Note the transition from thin-walled 

 cambium to the thick-walled tissue of the xylem. If the stem 



