ELM. STEM. 75 



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 Phloem, which is a much narrower band 

 than the xylem, is also marked off, though less distinctly, 

 into concentric rings of equal number. Outside this lie : 



4. The Cortical tissue and Cork, which are of 

 insignificant bulk, compared with that of the vascular 

 tissues. 



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. 



IV. Cut transverse sections from the above cut sur- 

 face, so as to include all the bands of tissue from the pith 

 to the cortex : moisten them with alcohol, and mount 

 in water or dilute glycerine. Examine with a low power. 



Note that the constituents of the several tissues, 

 produced during the later years, are similar to 

 those already observed in the first year's stem ; also 

 that they are arranged, more or less regularly, in radial 

 rows. This is best seen in the xylem : this points to 

 their origin from the cambium. 



Observe that the constituents of the autumn-formed 

 xylem are smaller, and have slightly thicker walls than 

 those formed earlier in the year, also that vessels of 

 large cavity are absent from it. Hence arises the 

 appearance of the annual rings. 



