VARIATIONS OF THE WATER-BALANCE. 65 



It is thus seen that the desiccation or reduction of the water-balance in 

 the Echinocactus may continue to such an extent that the specific gravity 

 and acidity of the sap is notably increased and the concentration of the sap 

 raised from 2 parts in 100 to 7 parts in 100, while the dissolved salts are 

 increased from less than 0.8 part to 3 parts in 100. The increase of con- 

 centration due to dissolved salts is therefore greater than that of other 

 dissolved material. The total concentration undoubtedly increases the 

 osmotic activity of the sap enormously. (See MacDougal and Cannon, 

 Conditions of Parasitism in Plants, Carnegie Institution of Washington, 

 Publication No. 129, p. 35, 1910 ) 



E CHINOCACTUS ( INVERTED ) . 



On January 28, 1909, an Echinocactus which was used for some experi- 

 ments in xeno-parasitism furnished data of interest in the present con- 

 nection. This plant had been freed from the soil a week previously, and 

 it was now brought in, set in a reversed position, with the apical end down- 

 ward, and the root- system removed by a cut which was made squarely 

 across the tapering base of the stem from which the roots arise. A cylin- 

 drical cavity was bored into the central cylinder of the stem, and into this 

 a clay bougie from a Livingston atmometer was fitted into the cavity 

 tightly with a thin solution of plaster of paris, the outer surfaces being 

 sealed with a thin coating of grafting-wax. A U-tube, of glass, with rub- 

 ber connections, was fitted to the cylinder, the whole system filled with 

 water, and the free arm of the U set in a glass dish containing a layer of 

 mercury, above which the water stood to a depth of several centimeters. 



The whole preparation gave opportunity for the absorption of water by 

 the tissues of the plant through the walls of the clay cylinder which would 

 draw a supply from the glass dish. The preparation being complete at 

 2 p. m., the free arm of the U-tube was thrust into the mercury, with the 

 result that at 3 p. m. a column 145 mm. had been raised in an hour. 

 Although the plaster had been allowed to set some few hours before, its 

 action was not entirely excluded. The column of mercury was allowed to 

 escape by raising the free arm above its surface to permit of absorption 

 and replacement of water. 



On January 29, 2 p. m., 50 c.c. of water had been taken up in 22 hours. 

 The free arm of the U-tube was lowered into the mercury for 3 hours, 

 during which time a column of the metal 120 mm. was raised. The arm 

 was again raised to allow absorption of water and various parasitic inser- 

 tions were made into the body of the plant, but these may be disregarded 

 in the present connection. The data given in table 23, on the following 

 page, were obtained from the preparation, corrections being made for possi- 

 ble evaporation from the dish of water from which the plant drew its supply. 



