ELM STEM 61 



b. Bast parenchyma : oblong cells with cellulose walls : some 

 contain protoplasm and starch, in greater or less quantity 

 according to the season : others contain crystals : note the 

 medullary rays as before. 



Passing inwards the differentiation of tissues of the phloem 

 is lost in 



6. The cambium, which appears here as a narrow band of 

 cells with thin walls, and abundant protoplasmic contents. The 

 form of the cambial cells is difficult to make out, but a careful 

 observation of a good section will lead to the conclusion that 

 the form of the cell as seen in the radial section is oblong and 

 very narrow, with square ends. Compare the diagrammatic 

 figure (p. 64). 



7. In the xylem, excluding for the present the medullary rays, 

 observe the following elements, all of which have lignified 

 walls 



a. Vessels of various orders, which may be grouped as 



(i.) Spiral vessels (protoxylem) found at the central part of 

 the xylem, next the pith : they are usually more or less dis- 

 organized, being often filled with thyloses. 



(ii.) Pitted vessels, the lateral walls of which are crowded 

 with bordered pits of essentially the same structure as those in 

 Pinus. These vessels are usually of large cavity. 



(iii.) Vessels with both pitted and reticulate marking, super- 

 posed on one another on the same lateral walls : these vessels 

 usually occur in groups, and are of small bore. 



Note in all these, but especially in (iii.), points where 

 transverse or oblique septa have been partially or completely 

 absorbed. 



b. Fibrous cells, which occur in large groups, between the 

 vessels : they are long, and prosenchymatous, and are inter- 

 twined, so that it is difficult to follow them through their whole 

 length. They have little or no cell-contents, and their walls 

 are not pitted. 



c. Xylem parenchyma : oblong cells with protoplasmic con- 

 tents, and starch : walls thick, lignified, and pitted : they occur 

 in longitudinal bands : note their close contact on the one 

 hand with medullary rays, on the other with vessels. 



