WATER-RELATION BETWEEN PLANT AND SOIL. 11 



absorption are entirely sound. It may well develop, however, that 

 the propagation of the "suction wave" from leaves to roots may not 

 always be so nearly instantaneous as the experiments of the last- 

 mentioned writer indicate, for it is well known that there are gas- 

 bubbles in some vessels of stems and roots. 



Certain phases of the general problem of plant water-relations have 

 been considered by Dachnowski,^ whose paper should be read in this 

 connection. 



The present paper deals with a first attempt to study the relation 

 between the diurnal march of the transpiration rate and the corre- 

 sponding march of the water-attracting power of the soil, in the case 

 of potted plants. The method employed is new and will require a 

 somewhat full description. 



METHOD. 

 APPARATUS AND MANIPULATION. 



The auto-irrigator, devised by Livingston" for the maintenance of 

 an approximately constant soil-moisture content, especially in the case 

 of plants growing in pots or boxes, was here employed. The device 

 consists of a porous clay cup, such as is now widely used in atmometry, 

 tightly stoppered and joined by a tube to a water reservoir at a lower 

 level, tube and cup being filled with water and the free end of the 

 former dipping below the water surface in the reservoir. The forces 

 of adhesion, cohesion, and surface tension (capillarity) draw water 

 from the cavity of the cup into its porous walls, in a manner exactly 

 similar to that involved in the original wetting of the cup walls in the 

 porous cup atmometer.^ If water is externally removed from the cup 

 walls more enters from within, the cavity of the cup being kept filled 

 with water by atmospheric pressure on the free water surface in the 

 reservoir; air does not pass the wet walls, excepting, of course, as it 

 dissolves in the contained water. In the porous cup atmometer 



'Dachnowski, A., Transpiration in relation to growth and to the successional and geographic dis- 

 tribution of plants. Ohio NaturaHst 14: 241-51. 1914. 

 ^Livingston, B. E., A method for controUing plant moisture. Plant World 11: 39-40. 1908. 



Hawkins, Lon A., The porous clay cup for the automatic watering of plants. Plant World 13: 

 220-27. 1910. 



Transeau, E. N., Apparatus for the study of comparative transpiration. Bot. Gaz. 52: 54-60. 

 1911. 

 'Babinet, J., Note sur un atmidoscope. Compt. Rend. 27: 529-30. 1848. 



Marie-Davy, H., Atmidometre ^ vase poreux de Babinet. Nouv. Met. 2: 253-4. 1869. 



Mitscherhch, E. A., Ein Verdunstungsmesser. Landw. Versuchsst. 60: 63-72. 1904. 



Livingston, 1906, page 24 et seq. 



, Operation of the porous cup atmometer. Plant World 13: 111-8. 1910. [This paper 



gives a Hst of hterature references, to which additions are made in the following two 

 citations.] 



, A rotating table for standardizing porous cup atmometers. Plant World 15: 157-62. 



1912. 



, Atmometry and the porous cup atmometer. In press. Plant World. 1915. 



Harvey, 1913. 



