Periodical Literature. 107 



reclly. or at least they can be used as a check upon the tempera- 

 ture summation method. The latter plan the authors carry out. 

 That is, they sum the normal daily mean temperatures of 106 

 stations in the United States for the period of the average frost- 

 less season (the direct index) ; they sum the temperature ef- 

 ficiences corresponding, respectively, to the normal daily means 

 and to the adopted coefficient (2) for each 10 degrees variation 

 (the efficiency index.) Then they plot both sets of tempera- 

 ture indices on a map of the United States, and the map is 

 then divided into areas by climatic lines in the usual way. To 

 compare the two series of indices thus charted, the ratio of 

 each direct index to the corresponding efficiency index was ob- 

 tained, thus giving a ratio for each station. These ratios were 

 also charted on a map. In a roughly approximate way the two 

 methods are in agreement, since for most of the area of the 

 United States they give results which agree within the limits of 

 a plus or minus variation no greater than 5 per cent. For local 

 areas, however, there are considerable variations — sufficient to 

 negate the correspondence of the two methods on an area which 

 one man would be likely to study in actual field work. The direct 

 index (summation) is a measure of the duration of the tem- 

 perature factor of climate, while the efficiency index (10 degree 

 variation) is a measure of the intensity of the temperature fac- 

 tor. Which of these more nearly approximates the measure of 

 the temperature effectiveness of a climate, so far as plant growth 

 is concerned, will no doubt remain for a long time undetermined. 



C. D. H. 



The Botanical Gazette, November, 1913, pp. 349-375. 



An extract from "Indian Engineering" 

 Forest brings up a controversy as to the value of 



Protection forests on steep slopes. One school has 



to assumed that the roots of forest growth 



Hill Slopes. extend into the crevices of rock and there- 



by assist disintegration and erosion. The 

 other school believes that the roots envelop the rock and soil 

 masses and "tie them together as a cord would do." Moreover, 

 it was argued that the action of water and frost was more se- 

 vere when there was no forest cover. The conclusion was 

 reached "There is no doubt that in the first place they never 



