﻿igi6] CURRENT LITERATURE 533 



branches on larger individuals growing in the open passed into a closed chamber 

 under a bell glass, where the amount of water given off by the plant was found 

 from the gain in weight of calcium chloride exposed within the chamber. Her 

 method of work, as described, seems to provide against any vitiating errors, 

 and clearly demonstrates in this shrub, both when leafless and in leaf, a distinct 

 drop and subsequent rise in actual and relative transpiration (transpiration 

 divided by evaporation) in the morning before the time of maximum evapora- 

 tion for the day. The maxima for relative transpiration for potted plants 

 were found to vary directly with the soil moisture, and some evidence was 

 obtained that the same variation exists in plants growing naturally in the desert 

 region about Tucson, Arizona. It was also found that Parkinsonia plants 

 in sunlight exhibit hourly changes in the relative transpiration rate, in the 

 amount of opening of stomata, in water content of leaves and twigs, and 

 in leaf temperature, and that these have evident interrelations which are held 

 to be governed by the ratio of the demand to the available supply of water. 



The transpiration absorption water balance in this woody perennial, as 

 it grows at Tucson, seems to be adjusted with the coming on of the summer 

 drought conditions by (1) the leaflets beginning to close earlier each day, until 

 finally they remain open only for a few minutes at dawn and twilight; (2) the 

 transpiration amount being lessened with the drying out of the soil; (3) the 

 leaflets and later the rachis dropping; and (4) the twigs and small branches 

 beginning to die. In addition to these seasonal changes, there is a daily closing 

 of leaflets and a lessening of actual transpiration rate, while the evaporating 

 power of the air is still increasing. This decrease is accompanied by a closure • 

 of stomata, a lowered water content of leaves and twigs, and a slight rise in 

 leaf temperature. The drop is followed by a rise, which, however, does not 

 equal the former maximum. Mrs. Shreve's investigation of Parkinsonia 

 may serve as a model for the study in detail of the transpirational behavior 

 of other similar and dissimilar plants in the accumulation of data necessary for 

 the understanding of the relationship of vegetation to atmospheric and soil 

 moisture. In attempting such studies, the investigators will do well to note 

 her conclusion that relative transpiration rates were found to differ according 

 to the previous environment of the plant, and hence that conclusions regarding 

 the actual transpiration of plants in situ cannot be drawn from the measure- 

 ment of losses from potted plants, but may be learned better from the measure- 

 ment from small branches of the plant grown in its natural environmenf. 



Among other recent contributions to our knowledge of transpiration is a 



a comparatively few experiments a comparison between rates of evaporation 

 and transpiration showed that here also a check in the latter process occurred 

 1-3 hours before the daily maximum rate of evaporation was reached, 



