SAP-WOOD AND HEART-WOOD. 



125 



for which their tubuhir and capillary character is especially adapted. 

 But the ducts in older parts, except when gorged with sap, contain 

 air alone ; and in woody trunks the sap continues to rise year after 

 year, to the places where growth is going on, mainly through the 

 proper woody tissue of the wood. In this transmission the new layers 

 are most active, and these are in direct communication with the new 

 roots on the one hand and Avith the buds or shoots and leaves of the 

 season on the other. So, by the formation of new annual layers out- 

 side of them, the older ones are each year removed a step farther 

 from the region of growth; or rather the growing stratum, which* 

 connects the fresh rootlets that imbibe with the foliage that elabo- 

 rates the sap, is each year removed farther from them. The latter, 

 therefore, after a few years, cease to convey sap, as they have long 



19S 



FIG. 196. Magnified cross-section of a portion of woody tis.«iue of White Oak, a year old. 

 197. A longitudinal as well as cross section of the same, a little higher magnified, a, a, Por- 

 tions of one of the smaller medullary rays. 



FIG. 198. Magnified cross-section of woody tissue from the same stem, taken from a layer 

 of heart-wood, 24 years old: b, ducts : a, portion of one of the minuter medullary rays. 199. 

 Combined cross and longitudinal section of the same : a, tissue of a medullary ray. 



11* 



