100 PRACTICAL BOTANY 



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 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 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. 



