ELM STEM 57 



least in winter). Tissue of this nature merges gradually into 

 the central tissue with thin walls (lignified and pitted), and no 

 protoplasm. Mucilage cells occur here and there. 



III. Cut a four-year old twig of Elm transversely, and smooth 

 the cut surface with a razor. 



The age of a twig may be judged externally by counting 

 backwards the annual increments of growth from the apex. 

 The limits of each annual increment of growth may be recog- 

 nized by the close aggregation of the scars of the leaves or 

 scales at those points. 



Examine with a lens, and observe 



1. The pith, which occupies the organic centre of the stem. 

 Its position does not, as a rule, coincide with the geometrical 

 centre. Externally to this lies 



2. The xylem, which is here a broad yellowish band, clearly 

 marked off into a succession of concentric rings ; these, as 

 a rule, correspond in number to the years of the twig (annual 

 rings). 



3. The cambium lies at the outer limit of the xylem, but it 

 will hardly be recognized as a definite band of tissue under a 

 simple lens, since it is a very narrow zone : its position may 

 frequently be recognized by the rupture of the tissues, the walls 

 of the cambium being thin and easily broken. Outside this 

 is 



4. The phloem, which is a much narrower band than the 

 xylem, and is also marked off, though less distinctly, into 

 concentric rings of equal number. Outside this lie 



5. The cortical tissue and cork, which are usually of 

 insignificant bulk, compared with that of the vascular tissues : 

 in some cases, however, there is an unusually great develop- 

 ment of cork, which then appears externally as longitudinal 

 projecting plates. 



Note the medullary rays. Some of these (primary rays; 

 may be traced the whole distance from pith to cortex ; others 

 (secondary rays) only part of that distance. The latter have 

 been entirely formed by the cambium. 



The general plan of the process of secondary thickening, 

 and the relation of the secondary tissues to the primary 



