PINE STEM 237 



similar in form to the sieve-tubes, whose cell-contents 

 are brown, and contain crystals. 



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 

 chlor-zinc- iodine), 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 the bordered pits, which are 



