PERIODICAL LITERATURE 737 



depth of two annual rings, or about ^ cm., and another group of five 

 trees of about the same normal yield of resin was cut 5 rings deep, or 

 Yo cm. The yield per face per "turn" for the first set was 15.8 grams, 

 as against 10.8 grams for the second set — a ratio of i to 0.68. In gen- 

 eral, for the first two years the cuts should not much more than open 

 up the surface, while later cuts should cut through as many rings as the 

 number of years the tree has been worked. 



5. Infuence of method of holding the adze. A cut perpendicular 

 to the bark gave 36 per cent less yield than an oblique (30°) cut and 

 30 per cent less than a flat (10°) cut. An oblique cut is harder to 

 make, since it is almost sure to go too deep : so the flat cut is consid- 

 ered best. 



6. Inftucnce of periodicity of i^'orking ( Nachdechseln) . The 1916 

 instructions of the Resin Committee called for a 4 to 5 day turn, while 

 the 19 1 7 instructions called for a 2-day turn. The writer says that 

 during the warm part of summer cuts should be repeated even more 

 frequently. Good yielding trees give up four-fifths of their flow in 

 from 2 to 3 hours. The period should be varied according to the 

 weather and time of the year. Possibly a three-day interval would 

 suffice early in the season, later reduced to two, and then to one in the 

 hottest part of the summer, and then gradually increased to three days 

 again. 



7. Influence of time of year, temperature, and weather conditions. 

 In spite of the fact that October, 191 7, was abnormally cool, moist, and 

 cloudy, with low atmospheric pressure, the yield of resin in that month 

 was almost as great as that in September, which was the highest month. 

 Good yields, better than any up to the middle of July, persisted until 

 the middle of November. The flow must, then, be influenced by other 

 factors to a much greater extent than by temperature conditions, which 

 by some have been considered the most important factor. The writer 

 believes that the production of resin is a by-product of metabolic assimi- 

 lation, which is most active in the fall of the year. 



8. Influence of the irritation caused by scraping. Although only a 

 small additional quantity of resin is obtained by scraping the cuts, it is 

 desirable to do this to keep the surface smooth and thereby insure 

 quicker flow and less loss by evaporation. Moreover, the scraping 

 seems to stimvilate the production of a secondary resin flow from patho- 

 logical resin ducts. This may be because it stimulates division of the 

 cambium cells, which, according to Mayr. causes the formation of resin 

 ducts, due to internal stresses and tension. The effect of the wound 

 irritation reaches, on the average, 6 cm. above and 2^ cm. below the 



