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



fested in the increased yields obtained by using the " advance streak." 

 (See pp. 10 to 12.) The effect of the wound on the actively growing 

 tissue is apparent in the tendency of the tree to heal or close the 

 wound. More than the normal amount of parenchyma or resin- 

 yielding tissue is formed, often at the expense of ordinary wood 

 formation. More resin passages or resiniferous parenchyma ag- 

 gregates were generally produced in a definitely limited region, 8 

 to 12 inches above the wound, than further above it, as is indicated 

 by the resin passage graphs in figures 1 to 6. Twenty or more 

 inches above the original wound the number of resin passages is 

 notably reduced, as is shown by the 1916 data as compared with those 

 of 1917 in figures 1 to 6. Nevertheless, the wound response giving 

 evidence of the extent to which the stimulus is transmitted, although 

 less marked at a distance, was manifested as far as 7 to 10 feet above 

 the wound. (PI. VI, figs. 1, 2, 3, and 4.) Both increase in resin- 

 iferous tissue, most marked in the transition wood and in the summer 

 wood in the specimens from the higher portion of the tree, and 

 often some reduction in ring width or summer wood formation, oc- 

 curred in the wood produced after turpentining. The chipping or 

 freshening of the wound is designed to remove the dried and hard- 

 ened surface of the streak and the unproductive parenchyma cells 

 in order to permit the fresh exudation of the oleoresin, which forms 

 and collects above this sealed surface during the period following the 

 chipping. The chipping also serves to stimulate the living cells 

 to further responses. It appears as if a very narrow chipping should 

 successfully accomplish this purpose. It is obvious that a high chip- 

 ping cuts away the most intensely stimulated and presumably the 

 most responsive tissue, especially for gum production, in the tree. 

 It is as if a whole organized battery of the tree's forces were wiped 

 out at each stroke of the hack and a new organization had to be 

 mustered afresh in the attempt to respond to the new condition. 

 After a number of such responses, the results of which are cut away 

 and wasted, the tree's resources tend to become more and more ex- 

 hausted, and the yield of gum and the wood formation are reduced. 

 Many trees under these circumstances become dry-faced — that is, 

 physiologically speaking, their living cells cease production, and 

 they frequently die (PI. VII, fig. 3) ; at best the vitality of the timber 

 is too severely taxed to assure the best returns possible. 



In brief, then, chipping in the lightwood (progressing up the tree 

 slowly less than one-half inch per week) is to be recommended be- 

 cause: (1) This is performed in the region of maximum stimula- 

 tion, (2) it conserves chipping surface, (3) it tends to keep the sur- 

 face from drying out because of the oleoresin saturating it to a 

 greater or less extent, and (4) it has been found experimentally and 

 practically to give sustained yields. Much " pulling " (see PL IV, 



