EXOGENOUS STRUCTURE. 



125 



more open and bibulous wood, which is apt to be surcharged with 

 sap, receive the name of SAP-WOOD, or ALBURNUM. The latter 

 name was given by the earlier physiologists in allusion to its white 

 or pale color. In all trees which have the distinction between 

 the sap-wood and heart-wood well marked, the latter acquires a 

 deeper color, and that peculiar to the species, such as the dark 

 brown of the Black Walnut, the blacker color of the Ebony, the 

 purplish-red of Red Cedar, and the bright yellow of the Barberry. 

 These colors are owing to special vegetable products mixed with 

 the incrusting matters ; but sometimes the hue appears to be rather 



an alteration of the lignine with age. In the Red Cedar, the deep 

 color belongs chiefly to the medullary rays. To show that the 



FIG. 161. Magnified cross-section of a portion of woody tissue of White Oak, a year old. 

 162. 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. 163. 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. 164. 

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

 11* 



