44 ECONOMIC WOODS OF THE UNITED STATES 



Roth, Filibeht; Timber, Bui. 10, U. S. Div. of Forestry, pp. 14-16. 



ZoN, Raphael: Methods of Determining the Time of Year at which Timber 

 was Cut, For. Quarterly, Vol. VIII, 1908. 



BucKHOUT, W. A.: The Formation of the Annual Ring of Wood in the Euro- 

 pean Larch and the White Pine, For. Quarterly, Vol. V, No. 3, Sept. 

 1907, pp. 259-267. 



Dacherowski, a.: Type and Variability in the Annual Wood Increment of 

 Acer rubrum, L. Ohio Nat. 8, pp. 343-349, 1908. 



NoRDLiNGER, H.: Die Holzringe als Grundlage des Baumkorpers, Stuttgart, 

 1872. 



Sanio, Carl: Botanische Zeitung, Vol. XXI, No. 50, 1863, pp. 391-399. 



Habtig, R.: Lehrbuch der Anatomie und Physiologic der Pflanzen, pp. 

 261-263. 



HEARTWOOD AND SAPWOOD 



The course of development of the various wood elements is 

 fundamentally the same, viz., they are formed in the cambium, 

 they increase in size, their walls thicken more or less, they function 

 as living cells for a time, but eventually lose their protoplasmic 

 contents and die. Their change from a living to a dead condition 

 is ordinarily not followed by immediate decay, and the cells 

 continue to perform the mechanical function of support. The 

 parenchyma cells remain alive for a longer time than the other 

 elements. 



The outer layers of growth of a tree, especially one of con- 

 siderable thickness, contain the only living elements of the wood 

 and comprise the sapwood. There is usually a sharp line of 

 demarcation between the living elements of the sapwood and the 

 non-living elements of the heartwood, though the vigor of the 

 living cells gradually wanes as their distance from the cambium 

 increases. Thejthickriess of sapwood varies widely in different 

 species, in different individuals, in different portions of a single 

 tree, and often on different radii of any particular section. Thin 

 sapwood is characteristic of certain genera, for example Catalpa, 

 RuMma, Toxylon, Sassafras, Morus, Gymnocladus, Juniperus, and 

 Taxus, while in others such as Hicoria, Acer, Fraxinus, Celtis, 

 and Fagus, thick sapwood is the rule. 



The fact that sapwood occupies the peripheral layers of the 

 stem causes it to form a considerable proportion of the volume. 

 The percentage of sapwood to total volume of the stem is for 

 certain species approximately as follows: Pinus palustris, 40; 



