ELM STEM 103 



in secondary phloem ; but their structure is often more 

 complicated, e.g. in Vitis or Tilia (see below, p. 116). 

 The sieve-tubes may easily be recognized in stems cut 

 in autumn by the masses of callus which surround the 

 sieve-plates : this stains brown with chlor-zinc-iodine. 

 For the reactions of the callus, see p. 115. Companion 

 cells are not easily seen. 



&. 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. 107). 



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 pirt of the xylem, next the pith : they are 

 usually more or less disorganized, being often filled 

 with thyloses. 



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

 crowded with bordered pits of essentially the same 



