70 



THE RELATION OF DESERT PLANTS TO 



OSMOTIC PRESSURE OF CACTUS JUICES. 



Attempts to express the juices from Boerhavia plants and determine 

 their osmotic pressure met with only indifferent success. The sap of 

 these plants is small in amount and very much thickened with slime- 

 like material, so that to express it in adequate amount for the determi- 

 nations was well-nigh impossible with the available apparatus. 



Better success attended similar attempts to determine the osmotic 

 conditions of the juices from the storage tissues of Echinocactus and 

 Cereus. The storage tissue was cut out in masses, chopped into small 

 pieces, mashed with a mallet, and then strained free from cells and 

 tissue fragments by means of a cloth filter. The extract thus obtained 

 was subjected to freezing-point determinations by means of the 

 apparatus of Beckmann. * The results of this determination are given in 

 Table XVII. Two tests of the freezing-point were made in each case 

 and their averages are used in the calculation of the pressures. 



TABLE XVII. Freezing-points of Cactus Juices. 



In the table, j denotes the lowering of the freezing-point, and the 

 calculated osmotic pressures at 25 C. are given in terms of atmos- 

 pheres, centimeters of a mercury column, and the pressure of a molec- 

 ular solution of a non-electrolyte, this being taken as 22.3 atmospheres 

 and denoted by M. 



A test of Echinocactus juice by the boiling-point method gave an 

 elevation of 0.08 C., and a calculated pressure at 25 C. of 3.6 atmos- 

 pheres, or 274.2 cm. of mercury, which is in very good agreement with 

 the results obtained from the freezing-point. 



The osmotic pressure of the cell sap of the cortex of Cereus was 

 determined also by the method, commonly used for such purposes, of 

 partial plasmolysis and variation in turgor tension. The epidermis and 

 the underlying storage tissue to a depth of about 5 mm. was removed 

 and cut into strips about 10 cm. long and 5 mm. wide. Owing to the 

 tissue tensions these immediately became concave on the epidermal 

 side, and the curvature was recorded by laying them upon paper and 



*For a description of the methods of freezing and boiling points here used, see 

 Livingston (1903), and references there given, or any book on physical chemistry. 



