Books and Current Literature. 195 



BOOKS AND CURRENT LITERATURE. 



The Physics of Transpiration. ^ — Students of plant 

 transpiration in its pyhsical aspect will be interested in 

 the recent studies of Renner '•' upon the influence of the 

 size, shape and position of an evaporating surface 

 upon its rate of water loss. This paper is a continuation of 

 the author's earlier studiesf upon the relation of form, position, 

 etc., of foliage leaves to their transpiration rates, with especial 

 reference to the vapor blanket which tends to cover foliar sur- 

 faces in quiet air. In the recent contribution, Renner is occu- 

 pied with physical evaporation from water surfaces and from 

 the surface of wet blotting paper. Pieces of the latter material 

 were supported on glass plates and care was taken that neither 

 too much nor too little water was added, to produce complete 

 saturation. After wetting the rate of evaporation remained 

 constant (under constant conditions) for an hour or longer, 

 during which time the experiments were carried out. The rate 

 of water loss per unit area was, in this first hour, not markedly 

 less than that from a free water surface. 



The following points are brought out:- Paper circles and 

 circular pans of water, placed upon a balance, lost about ten 

 percent more water when the balance was slowly and just 

 perceptibly swinging than when at rest. If the rate of evapor- 

 ation from horizontal paper surfaces be increased by wind to 

 six times its original magnitude, the original rate is regained 

 in not over two minutes after the wind ceases. Two parallel 

 vertical paper surfaces fail to influence each other sensibly 

 when but two cm. apart. When nine mm. apart their rate of 

 water loss is decreased only twenty-five percent. The water 

 vapor thus diffuses but little laterally from such surfaces. 

 Circular and rectangular papers lose moisture more rapidly 

 when vertically than when horizontally placed. Water loss 

 is more nearly proportional to the linear dimensions of the sur- 

 faces than to their areas, though various conditions modify 

 the proportionality. Of two rectangular surfaces with equal 

 areas but with different forms (in still air), the narrower always 

 loses considerably more water per unit of time than does the 

 broader. This is of great fundamental importance in comj arisons 



*Renner C, Zur Physik der Transpirati. n. Be", d d. Bot Ges. 29: 12135 2. 1911. 

 •fReuner, C. Beitratie zur Thysik der Transpiration. Flora, 100: 48S. l^JlO. 



