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1017] CURRENT LITERATURE 327 



In a more recent report by the same investigator, 9 the corrections pre- 

 viously indicated for humidity and light are used in experiments designed to 

 show the relation between the rate of transpiration and stomatal aperture, the 

 latter condition being determined by the use of the porometer. There was 

 also applied a further correction for cuticular transpiration. The final results 

 show many irregularities, but are regarded by Darwin as giving substantial 

 f support to his thesis that transpiration is regulated by stomatal aperture. He 



apparently finds nothing corresponding to the incipient drying of Livingston 

 and Brown, or the saturation deficit of Renner, although it seems possible 

 to the reviewer that some of his many irregularities might require some such 

 explanation. 



In striking contrast to this theory there comes an account of a study of 

 water relations of cacti. On account of their peculiar behavior, these plants 

 offer special advantages as well as special problems in the general study of 

 transpiration. Their transpiring power differs from ordinary plants in being 

 greater during the night than during the day. This behavior Mrs. Shreve 10 

 has investigated, and has found that there is a regular diurnal march of change 

 in the water-holding capacity of the internal tissues that seems both directly 

 and indirectly responsible for the changes in transpiring power; that is, the 

 transpiring power of the cactus is usually greater at night than during the day 

 because the water-holding capacity of the tissues is greater by day than by 

 night. The variations in water-holding capacity act upon transpiring power 

 indirectly by closing the stomata, which in cacti are usually closed during the 

 day and open at night, and they also act directly by resisting the evaporating 

 power of the air. It seems possible, as the author of this paper points out, 

 that a similar change in the tissues of non-succulents may account for the mid- 

 day drop in their transpiring power. 



Briggs and Shantz 11 have made an extensive and detailed study of trans- 

 piration as related to growth and to various climatic factors. The measure- 

 ments were made at Akron, Colorado, during the seasons of 19 14 and 191 5 

 and were for 270 pots of 115 kgm. each of soil, including some 25 different crop 

 plants. Continuous automatic records were obtained for air temperature, 

 solar radiation, wet bulb depression, wind velocity, and evaporation from both 

 shallow and deep tanks. Among the comparisons instituted, one of the most 

 instructive is the correlation between transpiration for the small grains and 

 various physical factors. This is expressed by mean correlation coefficients, 

 some of which are those with evaporation from a shallow pan, 0.87; with wet 



9 Darwin, F., On the relation between transpiration and stomatal aperture. 

 Phil. Trans. Roy. Soc. London B 207:413-437. 1916. 



10 Shreve, Edith B., An analysis of the causes 

 power of cacti. Physiol. Researches 2:73-127. 1916. 



* • 



11 



normal 



* ** r 7 7 — — ^ * 



period and its correlation with the weather. Jour. Agric. Research 7' *55~ 212 - J 9 l6 - 



