PERIODICAL LITERATURE 187 



measurement of the environmental factors, and thus deals with the 

 modern phase. The factors measured were : light, evaporating power 

 of the air, temperature, humidity, and wind. 



A brief survey is given of the recent previous investigations, in which 

 the work of some thirty investigators is analyzed and the result stated. 



The readings for the data on factors were made in the sun among 

 the leaves of the south periphery of isolated trees and at the apex of 

 trees growing in the forest. At a corresponding height to stui readings 

 on isolated trees, readings were taken in representative positions in the 

 crowns ; and for forest trees, readings were taken among the lowest 

 leaves. Care was taken to measure the different factors at the same 

 point in the tree. Cytological material was collected from both sun 

 leaves and shade leaves which showed the differences in structure. 

 The response of the leaves tmder these two different conditions was 

 shown in: the green and dry weights and water content of given leaf 

 areas ; the thickness of the leaf and its parts ; the compactness of the 

 tissues ; the structure of individual cells ; and in the macroscopic char- 

 acters, as area and lobing. 



In obtaining the physical factors, light w^as measured by the Clements 

 photometer between ii a. m. and 2 p. m. in Atigust. Livingston's stand- 

 ardized porous cups were used for measuring the evaporating power 

 of the air during the period from 8 a. m. to 5 p. m. Temperature w^as 

 taken by simultaneous readings in the two positions, and was usually 

 1° C. higher where the light was stronger. Wind velocity w'as also 

 obtained by simultaneous readings and by means of hand anemometers. 

 Humidity readings were obtained by means of cog psychrometers from 

 the same trees from which the temperature data were secured. 



]\Iuch original data are given on the effects of the physical factors 

 under the two contrasted conditions, set forth in tables and illustrated 

 by drawings of leaf sections. 



The article on the whole impresses one as the result of careful and 

 well planned work according to the best modern methods, and that it 

 has yielded results of value. The previous work is apparently well 

 canvassed and full use made of the data obtained and theories advanced 

 by other investigators. The results obtained are also thoroughly dis- 

 cussed and compared. A bibliography is appended and the essential 

 points summarized by the author as follows : 



I. The light intensity, as measured by the Clements photometer. 

 within the crown of 10 common broad-leaved trees was found in Au- 

 gust to vary from .0076 of full sunlight in Acer sacchaniiii to .1132 

 in Ou.rcus macrocarpa. 



