10 WATER-RELATION BETWEEN PLANT AND SOIL. 



Renner (1911), and Livingston and Brown 1 have shown that the 

 saturation deficit in the plant varies with the transpiration rate, and 

 the last-named writers have shown that the moisture content of 

 foliage leaves is relatively low in the latter part of the day (at least in 

 a somewhat arid climate) and very much higher in the later hours of 

 the night. The same has been shown by Lloyd. 2 Caldwell (1913) 

 appears to be quite correct in supposing that the state of incipient 

 drying must progress from the leaves toward the roots with the advance 

 of the day, and it follows from this that the highest magnitude of the 

 water requirement at the root surfaces should occur at a time somewhat 

 later in the day than that at which occurs the highest transpiration 

 rate. The lag thus postulated for the " Saugungswelle," or suction 

 wave as Renner has termed it, as the latter progresses from leaves to 

 roots, has been suggested by many studies of transpiration and absorp- 

 tion in the case of cut shoots absorbing from potometers. Edith B. 

 Shreve 3 has found incipient drying in the branches of a desert tree, 

 similar to that found by Livingston and Brown and by Lloyd in leaves. 

 So far as we are aware, however, Renner is the only writer who has 

 thus far actually dealt experimentally with the forces involved in the 

 absorption of water by roots and their relation to water movement 

 within the plant and water loss therefrom. In the first place, Renner' s 4 

 very ingenious experimental studies on water tension existing in ordi- 

 nary plants showed that the magnitude of this tension frequently 

 surpasses that of an atmosphere, even for low plants rooted in unusually 

 wet soil and exposed to the air of a rainy summer in Munich. Further- 

 more, the same writer 5 was able to demonstrate, not only that this 

 tension is transmitted to the roots, but also that comparatively small 

 alterations in its magnitude (resulting from changes in the transpira- 

 tion rate or in the resistance to water absorption) are so propagated. 

 His experiments lead Renner to the conclusion that the absorptive 

 power of the roots is due to tension thus transmitted from the transpir- 

 ing parts. This is exactly what should be expected from the existence 

 of tension in the water mass of the plant body (arising from whatever 

 cause and in whatever region) and from the consideration that such 

 tension in a liquid should be transmitted equally in all directions, 

 as far as the liquid extends. It seems to the present writers that 

 Renner's conclusions in regard to the general mechanics of water 



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



in the water content of foliage leaves. Bot. Gaz. 53: 311-30. 1912. 

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



leaves of Fouquieria splendens. Plant World 15: 1-14. 1912. 

 3 Shreve, Edith B., The daily march of transpiration in a desert perennial. Carnegie Inst. Wash. 



Pub. 194. Washington. 1914. 

 4 Renner, O., Versuche zur Mechanik der Wasserversorgung. 1. Der Druck in den Leitungs- 



bahnen von Frielandpflanzen (Vorlaufige Mitteilung). Ber. deutsch. Bot. Ges. 30: 



576-80. 1912. 

 Renner, 1912 (2). 



