144 RESEARCHES ON FUNGI 



osmotic pressure of approximately 24 atmospheres,^ and from it 

 a series of other solutions was made, these solutions having 

 osmotic pressures of approximately 12, 10, 9, 8, 7, 6, 5, 4, and 3 

 atmospheres. 



As a result of making five sets of experiments on five different 

 occasions, the sugar solutions having been freshly made and the sap 

 freshly collected on each occasion, it was found that the osmotic 

 pressure of the cell-sap of Pilobolus longipes is less than 24, 12, 

 10, 9, 8, 7, and 6 atmospheres and greater than 3, 4, and 5 atmo- 

 spheres. We may therefore take it that the osmotic pressure of 

 the cell-sap of P. longipes is approximately equivalent to 5-5 

 atmospheres. 



The collective work of various investigators has shown that the 

 osmotic pressure of the cell-sap of the higher plants varies from about 

 3-5 atmospheres in the leaves of Aloe americana grown in a green- 

 house to upwards of 100 atmospheres in the leaves of plants growing 

 in the rocky desert of the Sahara ^ and to a maximum of about 170 

 atmospheres in the leaves of Atriplex nuUallii growing in the desert 

 areas of Utah (U.S.A. ).2 For the sake of comparison with Pilobolus, 

 a series of osmotic pressures determined by Dixon and Atkins 

 will now be cited. Dixon and Atkins * treated the leaves of a 

 number of different kinds of plants with liquid air, pressed out 

 the sap, measured the freezing point of the sap, and then calcu- 

 lated the osmotic pressure of the sap. Their findings, showing 

 the range of the osmotic pressures, have been embodied in the 

 adjoining Table. 



Tjrp 



^ This pressure was calculated from the equation : P = — — , where P is the 



osmotic pressure in atmospheres, R the gas constant (0-082), T the absolute 

 temperature (293° C), g the grams of sucrose per litre of solvent (342), and M the 

 molecular weight of sucrose (342). Vide J. C. Phillip, Physical Chemistry, Its 

 Bearing on Biology and Medicine, London, 1925, p. 55. 



^ Hans Fitting, " Die Wasserversorgung und die osmotischen Druckverhaltnisse 

 der Wiistenpflanzen," Zeitschrift fiir Botanik; Bd. Ill, 1911, pp. 270-271. 



^ J. A. Harris, R. A. Gortner, W. F. Hoffman, J. V. Lawrence and A. T, 

 Valentine, " The Osmotic Concentration, Specific Electrical Conductivity, and 

 Chloride Content of the Tissue Fluids of the Indicator Plants of Tooele Valley, 

 Utah," Journal of Agricultural Research, Vol. XXVII, 1924, p. 909. 



* W. R. G. Atkins, Some Recent Researches in Plant Physiology, London, 1916, 

 pp. 154-155. 



