62 TRANSPIRATION IN A DESERT PERENNIAL. 



maximum water loss for this plant, under the existing conditions of soil 

 moisture and water content of tissues, is being reached, and it is probable 

 that the more severe conditions of midsummer would cause a more pro- 

 nounced drop. It will be noted that the maximum transpiration amount of 

 No. 7 on April 11 seems to bear somewhat the same relation to its limit as 

 does that of No. 6 on the same date, although the evaporation conditions 

 were very different. The facts that No. 7 received water just preceding the 

 sealing and that No. 6 had been without water for 48 hours make these 

 results agree in a general way with the results of experiment X. 



DISCUSSION OF LITERATURE. 



Lloyd* has taken simultaneous readings of transpiration, evaporation, 

 water content of leaves, and stomatal movement, but, while his read- 

 ings doubtless answer satisfactorily the question which he had in mind, 

 they are of no avail for discussion in this paper because of a haziness in the 

 sky which he records as existing between 8 a. m. and 10 a. m. 



Livingston and Brown,f find in general that green plants, in the desert 

 of Arizona, "exhibit a marked fall in foliar moisture content by day and 

 a corresponding rise by night," and they give as the probable cause of this, 

 " incipient drying brought about whenever the ratio of water loss to water 

 supply in the leaves is rendered less than unity." Unfortunately the above 

 paper had not appeared when the greater part of the present work was carried 

 out, and so it could not be taken into account in the planning of the experi- 

 ments. Livingston and Brown have evidence which leads them to suggest that 

 small-leavedxerophytesshow a " higher water content by day than by night." 

 The present work shows that this is not true in the case of Parkinsonia, for 

 in addition to the hourly curve of leaf moisture given, three different tests, 

 made on trees on different days, show the lowest daily water-content at mid- 

 day. On the whole it may be said that the present paper offers additional 

 evidence for Dr. Livingston's theory of "incipient drying" of leaves and 

 shows that the same phenomenon takes place also in stems. 



* Lloyd, F. E., The relation of transpiration and stomatal movements to the water 

 content of the leaves of Fouquieria splendens. Plant World, vol. xv, p. 1, 1912. 



f Livingston, B. E., and Brown, W. H., Relation of the daily march of transpiration 

 to variations in the water content of foliage leaves. Bot. Gaz., vol. LIH, p. 311, 1912. 



