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 (/?, 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 
(c7F, 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. 
