ELM STEM 59 



arrangement are made clear by means of the diagrams A, B, 

 C of Fig. 6. 



IV. Cut thin sections from the transversely cut surface, 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 gly- 

 cerine. 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 regu- 

 larly, in radial rows. This arrangement is best seen in the xylem, 

 and it points to the origin of the tissues 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, 

 which is easily seen in almost any wood of temperate climates, 

 when the transverse section is examined. 



In arboreous stems of considerable age a central portion of 

 the woody mass (duramen, or heart- wood) becomes differentiated 

 by texture and darker colour from the more superficial and later 

 formed portion (alburnum, or sap-wood) : this may be seen in 

 old stems of the Elm, but more distinctly in stems of the 

 Laburnum, where the heart-wood is coloured dark brown, while 

 the sap-wood is light yellow. Compare also the black duramen, 

 and light-coloured alburnum of Ebony, and the red duramen of 

 the Logwood, &c., &c. 



As stems grow older, layers of cork appear successively further 

 and further from the external surface : not only the cortex but 

 also the outer and older portions of the phloem are thus cut off 

 from physiological connection with the inner tissue : the term 

 bark is applied to tissues thus cut off, together with the cork 

 which forms the physiological boundary. 



V. Cut radial sections from a four-year old stem of Elm ; 

 soak them ten minutes or more in alcohol to remove the air- 

 bubbles, and mount in glycerine. Other sections may be 

 mounted in Schulze's solution, and these will perhaps be found 

 the most useful ; examine first under a low power. 



It will be found difficult to cut good sections so as to include 



