4i8 SCIENCE PROGRESS. 



find some fundamental plant process in need of assist- 

 ance. 



Such a process is transpiration. Experiments made 

 with branches of beech and hazel, bearino-, in the case of 

 either plant, red and green leaves respectively, show that 

 the transpiration of the red leaves is greater than that of 

 the green, especially if the branch be not exposed to 

 direct sun, or if it be kept in an atmosphere rich in 

 aqueous vapour ; which conditions, it is to be noted, are 

 those imposed by their habitat on shade-loving tropical 

 plants. 



Other experiments made with branches bearing red- 

 spotted leaves, e.g:, Polygonum persicaria, Maranta Ker- 

 choviaiia, in which the cut stalks were placed in eosin 

 solution, showed that the liquid travelled first to the red 

 patches of the leaf, from which it is reasonably inferred 

 that transpiration is greatest in those areas. These experi- 

 ments are the starting-point for Stahl's most interesting 

 speculations. He recalls Wiesner's {^^;i^) discovery that 

 plants in a saturated atmosphere may continue to give off 

 water of transpiration owing to the conversion of light rays 

 by chlorophyll into heat, and a consequent increase in 

 tension of the aqueous vapour of the inter-cellular spaces in 

 the plant above that of the vapour outside. Stahl reasons 

 that anthocyan as a heat absorber may act in a precisely 

 similar way and supplementary to the chlorophyll. Now 

 there is indeed some evidence to show that transpiration of 

 plants within the tropics may, under certain conditions, be 

 very low. Haberlandt (34), whilst in Java, made experi- 

 ments which gave the remarkable result that transpiration 

 of plants belonging to the middle Europe Bora is about 

 half as much in the tropics as it is in their native habitat. 

 Stahl has criticised these experiments adversely. Burger- 

 stein (35) and Wiesner have shown conclusively that 

 Haberlandt's results give a most misleading picture of 

 normal transpiration within the tropics, since he excluded 

 direct sunlight from his plants. Into these matters, how- 

 ever, we need not enter here, since Haberlandt's experi- 

 ments, under the conditions of his 7'esearch, are undoubtedly 



