154 The Plant World 



with potassium nitrate solution in the usual manner. The solu- 

 bility of potassium nitrate limits the possible osmotic pressure of 

 an aqueous solution of this salt to about 100 atmospheres * 

 and it ^vas found impossible in many cases to obtain plasmoly- 

 sis with the saturated solution. In certain of these cases a 

 saturated solution of sodium choloride, containing 5.42 gram- 

 molecules per liter, produced plasmolysis. I 



As many plants as possible, from a number of different 

 soils, were tested to determine the influence of the latter (espec- 

 ially in regard to their moisture content) upon the magnitude 

 of the foliar osmotic pressure. In many instances it was possi- 

 ble to test the same species on several soils. The main soil 

 types considered were: The mountain desert (Felsen- 

 Wiiste) of the Chaine de Sfa, the boulder desert (Gerollwiiste) , 

 dry soil, formerly cultivated but now abandoned (trockenes, 

 wiistenahnliches Kulturland), sand dunes, saline swamps 

 and moist, non-saline soil (either irrigated of near water in 

 the Oued Biskra. The results obtained are so important, not 

 only with regard to the possible magnitudes of plant osmotic 

 pressure, but also with reference to the relation of this pressure 

 to environmental conditions, and to our general knowledge of 

 the fundamental water relations of plants, that I have selected 

 from Fitting's tables a few of the observed pressures for pre- 

 sentation here. The e are given below, the quantities being 

 all in terms of gram-molecules of potassium nitrate per liter,** 

 and all referring to the leaf epidermis. In many cases the 

 mesophyll was found to exhibit a somewhat higher pressure than 

 the epidermis. In the mountain desert and in the boulder 



desert the conditions, both of plant and sziil are in close agree- 

 ment, so that I have not deemed it worth while to quote separate 

 examples for these two soil types. For such details, as well 

 as for determinations upon many more forms, the reader 

 must consult the original paper. 



♦The most concentrated solution used by Fitting was saturated at 22o C, and contains 

 2. "50 gram-molecules per liter. This concentration is assumed as 3-normal. with a pressure 

 of about 100 atmosphetcs. 



tThis method of using as a basis for dilution a solution saturated at a given tempera- 

 ture. avoiJiug .as it does the use of the balance, has much to reccommend it for field studies 

 which require the preparapration of salt solutions. There is no advantage in haying the 

 stock solutions, from which dilutions are to be made, of any integral number of gram- 

 mo'ecules per hter. 



* *These may be reduced, approximately, to atmospheres by multiph-ing by 36.9, 34.07, 

 or 33.3. according as the obser\-ed isotonic solution approaches respectively! 1.0, 2.0, oi 

 3.0 gram-molecules per liter ^see original paper). 



