CURRENT LITERATURE. 



Der Lichtgenuss der PHansen. Von Professor J. Wiesner. 

 Leipzig. 1907. K. 10.80. 



In connection with the highly interesting data furnished by 

 Cieslar in regard to tolerant and intolerant species, briefed on 

 another page, we may appropriately refer to the same author's 

 review of Wiesner's special work on the light requirements of 

 plants in general, a synopsis of 15 years of study in this field in 

 which he is undoubtedly leading authority. The subject is treated 

 in eleven chapters, and brings, besides references to previous 

 work of the author and of others which in the photometric direc- 

 tion began in 1877 with a forester, Theodor Hartig, also the re- 

 sults of entirely new investigations. 



The first chapter treats of photometric methods; the second 

 analyzes the daylight, direct and reflected. 



A spectroscopic investigation of the light at the exterior and 

 in the interior of densely shading crowns revealed that down to 

 a decrease of the light in the crown to 1/80, i. e. down to practi- 

 cable limits, a change in spectral composition could be discovered 

 by Wiesner's method. 



Every plant adapts its organs to the light conditions sur- 

 rounding it, either by defending itself against surfeit or by pre- 

 paring for full utilization. This adaptation he calls "Lichtraum- 

 nutzung" — utilization of light space. Thus, a tangential plane 

 laid around a tree crown, which Wiesner calls the maximum light 

 plane, is the measure of the light which is available to the plant. 

 According to Wiesner, under natural conditions the total leaf 

 surface of a tree is as a rule smaller than this plane — a rather 

 astonishing fact. For beech, a tolerant species which has a mini- 

 mum light sufficiency (Lichtgenuss-minimum) of 1/60, the leaf 

 surface is .8 of the plane ; for spruce with a light sufficiency of 

 1/30, it is .5, for larch with 1/5 light sufficiency, .2. 



The Lichtgenuss is the relation of the intensity of the light 

 reaching the plant to the intensity of the total daylight — a photic 

 ratio — and is expressed in any convenient photometric unit. In 

 the fourth chapter this Lichtgenuss which we may render into 



