PINUS. STEM. 143 



Note on passing to the periphery of the phloem an 

 increasing irregularity of form of the tissues due to 

 distortion, caused by pressure from without by the 

 cortical tissue upon the vascular system, as it increases 

 in bulk by secondary thickening. 



Sclerenchymatous elements are absent from the phloem of the 

 stem of P. sylvestris. They are, however, found in the phloem 

 of many of the Coniferce, e.g., Juniperus, in which the different 

 tissues are arranged with great regularity. 



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 was cut in winter the 

 transition will appear sudden, if cut in summer it 

 will appear gradual. The tissue-elements retain the 

 same arrangements in radial rows, as the cells of the 

 cambium. 



Observe that the xylem ring is cut by the medullary 

 rays into wedge-shaped areas. The chief tissue-elements 

 filling these areas are the Tracheides, which present 

 the following characters : 



i. They have approximately the same shape as the 

 cells of the cambium from which they are derived. 



ii. Their walls are thick and lignified (yellow with 

 Schulze's solution), and are differentiated into layers 

 distinguished optically, and by staining. 



iii. They have no cell-contents. 



iv. On their radial walls (and rarely on the tan- 

 gential walls) are found irregularities of structure called 

 Bordered pits, which are best seen in the xylem formed 

 at the early part of the year. Note the pit-membrane, 



