6 BULLETIN 1064, U. S. DEPARTMENT OF AGRICULTURE. 



the older cells tend to become more or less impregnated with resin 

 as they are transformed into heartwood, which serves primarily for 

 mechanical support. The resin in the heartwood flows out much less 

 easily than that in the sapwood, as may be seen on any freshly cut log 

 or stump. Although the heartwood contains more resin by chemical 

 analysis, it will not, if tapped, produce a sustained yield of resin, as 

 the sapwood, if properly treated, will. Gum turpentine is obtained 

 from a number of the outermost sapwood rings (PL III, fig. 2) . The 

 depth of the cut in relation to the width of the sapwood is a very 

 significant factor with reference to both the yields obtained and the 

 injurious effect of turpentining upon the tree. Successful turpentine 

 operations depend upon keeping the sapwood in a healthy con- 

 dition, capable of responding not only to normal stimuli but to the 

 special wound stimuli as well. 



TRACHE1DS. 



During the growing season, April to November, the individual 

 cells of the outermost annual ring of the sapwood, as they "are de- 

 rived from the cambium, become differentiated and develop into 

 specialized structures. These are the tracheids. (T, PL II, figs. 

 1 to 5.) They are vertically extended cells which complete their 

 formation and lose their protoplasmic content early. They then 

 serve for sap conduction and finally for the mechanical support of 

 the tree. They are not directly connected with oleoresin production, 

 although they constitute a significant part of the organic mechan- 

 ism of the tree. They differ somewhat in appearance, as was stated 

 in the discussion of spring wood and summer wood (p. 5). The 

 character of the summer wood tracheids and also the total number 

 of tracheids formed often serve as indicators of the vitality and 

 responsive power of the trees under observation. 



PARENCHYMA. 



The parenchyma tissue found in pines may be considered under 

 two main heads: First, the horizontally extended parenchyma cells 

 constituting the rays (i?, PL II), including the fusiform rays (FR, 

 PL II, fig. 4) with their horizontal resin passages ; second, the verti- 

 cally extended parenchymatous elements, which grouped together 

 (GP, Pis. II, V, and VI), form the resin-producing centers, the 

 so-called resin passages, ducts, or canals. The parenchyma cells 

 are characterized by the fact that they " remain alive," that is, re- 

 tain their protoplasmic content and function actively for a number 

 of years or until approximately the time when the inner rings of 

 the sapwood change into the heartwood. 



