PERIODICAIv LITERATURE 731 



shade, dense stands, dense undergrowth, and cold sites tend to cause a 

 small flow. The pressure of the outer air tends to counteract the cell 

 pressure ; hence on sultry days, when the barometer is low, the resin 

 flow is apt to be copious. Fluctuations in barometric pressure prob- 

 ably partly explain the variations in rate of flow on the same day. 



Boxing cuts off the water from below while the drain from above 

 continues, so tends to diminish the flow of resin. The gutter system 

 also has a slight effect of this sort. The boring system, which uses 

 glass flasks inserted in auger-holes to catch the resin, is rather imprac- 

 tical because of the cost of the flasks (particularly under war condi- 

 tions) and the necessity, of boring a great many holes, which can be 

 used but once in each tree. It may be applicable where a particularly 

 volatile resin, rich in turpentine, is obtained, as in the case of silver fir. 

 Tubeuf prefers the system which uses metal strips, either nailed to the 

 tree or provided with prongs which can be inserted under the bark or 

 under paper strips tacked to the bark. He says that these do not inter- 

 fere at all w^ith the flow of resin, w'hile gutters which are driven into 

 the wood do so. 



Dr. IMiinch, technical adviser to the Resin Section of the War Com- 

 mittee for Oils and Fats, gives the following data on resin yield of the 

 Scotch pine. He uses as a unit for measuring yield the weight of resin 

 obtained from one cut embracing one centimeter of the tree's diameter, 

 wounded once only. Much of the available data on yield are worthless 

 because no allowance is made for the number or breadth of cuts or the 

 number of times that the resin was collected during the season. The 

 depth of cut is not important, since although deep cuts may yield more 

 at first, in the long run they yield only as much or even less resin than 

 shallow cuts. Ordinarily the resin flows for about 24 hours after the 

 cut is made, and another 24 hours is required to fill the canals and 

 restore internal pressure. The cutting can therefore be repeated every 

 two days, and nothing is gained by waiting longer. In his tests the 

 average yield per hundred faces (average width, 12.5 cm.) was 0.89 kg. 

 with a two-day interval between cuts, and 0.88 kg. with a three-day 

 interval. The scrape was not included, but was estimated at about 0.3 

 kg. in each case; so that the total resin yield was 1.2 kg. per 100 cuts, 

 or 12 g. per cut, or i gram per centimeter of circumference. This 

 figure, which agrees fairly well with results obtained by other investi- 

 gators, is the average for Scotch pine in the German pine region. In- 

 dividual trees of course vary greatly in their yields, because of differ- 

 ences in climate, soil, site, time of year, weather, size of trees, skill with 

 which the work is done, and other less well understood factors. The 



