74 CARNEGIE INSTITUTION OF WASHINGTON. 



months and is more abundant in some years than in others. The in- 

 fluence of this sap, and of the pressed juice of leaves, on the imbibition of 

 agar and of gelatine has been investigated. The total imbibition of water 

 by agar was diminished by the pure juice and by the dilution used, 

 the effect of juice from the xerophytic leaves being the greater. In 

 the case of gelatine total imbibition was increased by the juice from 

 the mesophytic leaves, but markedly diminished by that from the 

 xerophytic ones. Measurement of the total acidity showed that no 

 causative agency existed in the changes in acid content. 



The influence of the juice on the retentiveness of water by agar jelly 

 was tested by making a jelly of agar with undiluted juice. An appara- 

 tus was arranged so that an altered porous-cup atmometer could 

 draw water simultaneously from the two kinds of jellies. The large 

 opening of a thistle tube was covered with thin white silk and the bulb 

 embedded in the jelly in a tall stender dish, with the tube passing 

 through a cork and projecting above the dish. Two dishes, one con- 

 taining agar made with the juice and the other with water, were con- 

 nected above to the atmometer cup. Each dish with its thistle tube 

 could be removed and weighed separately. The water-loss from the 

 agar-water jelly was to that from the agar-juice jelly as 1.00 to 0.67. 

 The experimental error was too great to detect an undoubted differ- 

 ence between the juice from the two types of leaves, but it was very 

 clear that the juice at both seasons retarded the water-loss of agar. 



Two other methods were used to test the validity of the results. In 

 one case blocks 10 by 10 by 1 cm. were cut and allowed to lose water 

 slowly by evaporation in a closed chamber; in the other, cubes (2 cm.) 

 were immersed in concentrated sugar solution. In both cases the agar- 

 juice jelly lost less than the agar-water jelly. These results were 

 very surprising, since it had been supposed on a priori grounds that a 

 substance which decreased the total imbibitional capacitj^ of a colloidal 

 jelly would also decrease the retention of water against an external 

 agency, particularly if the jelly were made to contain more water than 

 it could imbibe si the temperature under consideration. The juice 

 contains a mixture of many substances, and it was consequently decided 

 to withhold conclusions until a separate investigation could be made 

 using single substances. 



The effect of the juice of Encelia farinosa on the retention of water 

 by agar jelly made it seem advisable to undertake a separate investi- 

 gation to determine whether or not a substance which changes the 

 imbibitional capacity of a colloidal jelly changes also its resistance 

 to water-loss by an external agent, and, if so, what relation exists 

 between these changes. Preliminaiy experiments have been made 

 with formic acid and with tyrosin, using the arrangement of thistle 

 tubes and atmometer cups described in a previous paragraph. The 

 results thus far indicate that these two substances, which decrease the 



