Atkins — The Hydrogen Ion Concentration of Plant Cells. 421 



Hydrogen ion concentrations of plant secretions and exudations. 



It seemed of interest to study the reaction of the transpiration stream, 

 since, owing to the fact that it is evaporated in the leaves, the acidity, if any, 

 would be concentrated in course of time. The determination is not easily 

 made, since even if a woody stem is cut and centrifuged (Dixon and Atkins, 

 1915) there is the risk of contamination with the sap of living cells, and this, 

 though diluted, might cause an appreciable error. It is hoped, however, to 

 test this by direct experiment, and to see whether the variation in sugars 

 previously shown is accompanied by any change in acidity. 



Eecourse was had to the liquid which drips from the leaf tips of Colocasia 

 antiquorum. Attention had been drawn to the great purity of this water by 

 early workers, Duchartre (1859) and Musset (1865), and a plant growing in 

 the Trinity College, Dublin, Botanic Gardens was found by Dixon (1914) to 

 have a freezing point indistinguishable from pure water when tested by the 

 thermoelectric method, one couple being in each of the two liquids. Further, 

 the electrical conductivity was less than that of Dublin tap-water, a very soft 

 upland water from a granite and quartzite area. The solids amounted only 

 to 0-012 per cent, as determined by evaporating 20 c.c. of the liquid. It had 

 also been shown by Miss Flood (1919) that this liquid is in direct connexion 

 with the conducting tracts of the leaf, and has therefore passed through living 

 cells only in the roots. The colorimetric determination with brom thymol blue, 

 using 10 c.c. of the liquid which issues from the leaf tips, gave pH 6-8, which 

 is a value given by distilled water quite frequently owing to the absorption of 

 carbon dioxide. With the same indicator it was found that sap pressed from 

 the leaf stalk was at pH 5-4, and the leaf tissue at 6-0. It is clear, therefore, 

 that this water of the transpiration stream is entirely uncontamiuated with 

 acids, though it has passed through the root tissues and the conducting tracts 

 of the entire plant. 



Another liquid of interest is the water found in the axils of the leaves of 

 Bipsacus laciniatus. This has been shown by L. B. Smyth (private com- 

 munication) to be without digestive action. A plant growing in the Botanic 

 Gardens was found to contain much water in the axils. This was at pH 6-8 

 in a lower leaf and 7'0 in an upper one. The lower one contained dead flies. 

 Since the water was present after a considerable period of drought, it is 

 clearly derived from the plant, and is quite possibly in connexion with the 

 conducting system. This point was not, however, investigated. 



The pitchers of Sarracenia spp. are also well known to contain liquid. It 

 is stated that this is without digestive action, but that insects caught in it 



