Atkins — Osmotic Pressures of some Plant Organs. 465 



rough idea of the chemical substances present in predominating proportions 

 may be got from a knowledge of the mean molecular weight of the sap 

 solutes, this value has in many cases been determined. To accomplish this 

 it is necessary to evaporate a weighed amount of filtered sap in a steam-oven 

 till a constant weight is obtained. The quantity used should not be above 

 three grams, or great difiiculty will be experienced in completely drying the 



syrupy liquid. The calculation is made from the formula: — M = r ^ "T' 



where M denotes the molecular weight to be determined, I the weight of the 

 solvent, viz. the weight of the sap minus the weight of the solids, s ; A 

 denotes the depression of freezing-point ; and k is the constant for the 

 molecular lowering of freezing-point of the solvent, water in this case. 



Discussion of Results. 



On taking a general survey of the pressures recorded, it is seen that they 

 range from 5-94 atm. for a specimen of the fruit of Lycopevsicum esculentum 

 to 29-53 atm. for the fruit of Pyrus communis. In most instances a number 

 of results are recorded for various samples of the same organ, as it was 

 uncertain what degree of constancy of osmotic pressure was to be expected. 



The value 9-26 atm. for Ruhus Idceus is the mean of the pressures of 

 many ripe berries. Cucumis Melo, a fruit ripe for eating being used, gave a 

 slightly higher pressure, 9'86 atm. 



Pntnus communis, mature and very sweet fruits ranged from 28-27 to 

 29-53 atm. The large amount of sugars probably accounts for the high 

 pressure. Two or three greengages were pressed to afford the juice in each 

 case. This gives reason to believe that fruits of the same species in the same 

 condition of ripeness have roughly the same osmotic pressure. 



Citrus Limonum (fruit) gives larger variations, 10-46 to 12-21 atm. in 

 samples bought in August, and 12-05 to 14-06 atm. in those purchased in 

 February. It is to be noted that the pressure in the rind, 11-53 atm., is 

 lower than that in the pulp of the same fruit, 12-05 atm. The mean 

 molecular weight, ranging from 145 to 160, points to organic acids as the 

 chief agents in producing the pressure, for any considerable quantity of 

 dextrose, levulose, or cane-sugar would considerably raise the mean. In this 

 fruit citric acid, mol. wt. 192, is known to be present in quantity, and its 

 salts, which are ionized in solution to a much greater extent than the acid 

 itself, would tend to reduce the mean molecular weight. 



Ripe fruits of Citrus Aimmtium in August gave pressures from 16-84 to 

 19-22 atm. ; while the more acid fruits in February had a lower pressure, 

 12-27 to 12-40 atm. The last-mentioned sample gave as mean molecular 



3t2 



