OSMOTIC PRESSURE IN THE CELL 



141 



relatively attenuated. Judging by this method, more than half the osmotic 

 pressure of the beet-root has been found to be due to cane-sugar, and 

 a similar proportion of that of the onion to glucose, which latter may be 

 responsible for as much as eighty per cent, of the pressure of turgor in 

 rose petals. In the leaf-stalks of Gunnera scabra about fifty-four per cent, 

 is due to KC1, and in the pith of the shoot apex of Helianthus tuberosus 

 about forty-one per cent, is due to KNO 3 *. In the leaf-stalk of Rheum, 

 oxalic acid is of primary importance (accounting for sixty-two per cent.). 

 The same is also the case in Oxalis, while many Crassulaceae accumulate 

 large amounts of malic acid 2 . 



High turgidity is a necessary result of the accumulation of soluble 

 osmotic reserve-materials, but it is of subordinate importance only. In 

 other cases, as for example in growing cells, the maintenance and regulation 

 of turgor are of the utmost importance, and for this purpose the plant 

 appears preferably to employ organic acids, although sometimes other 

 substances, and even inorganic salts, may be utilized. 



The production of organic acids appears to be especially favourable for 

 the maintenance of turgor, for although during the oxidation of dextrose 

 to citric acid the osmotic pressure remains the same, when dextrose is 

 oxidized to malic or oxalic acid, an increased osmotic pressure is generated, 

 while at the same time energy is liberated 3 . 



The union of the acid with an alkaline base still further increases 

 the osmotic pressure, which, however, is very slightly if at all modified 

 by combination with an alkaline earth. 



In the economy of the plant, according to the character which meta- 

 bolism assumes, it is of great importance that substances of lower or of 

 higher osmotic powers may be produced from the same nutrient material, 

 and a knowledge of the osmotic powers of secreting cells will aid us in 

 establishing conclusions as to the causes of passive, or even active, secretion. 



In an insoluble form (as starch, oil, or proteid), reserve-material may 

 be stored up without causing any increase of osmotic pressure, and even 

 by the mere condensation of two molecules of a monosaccharide to one of 

 a disaccharide (cane-sugar), the osmotic pressure may be reduced to one 

 half of what it previously was. 



If the sugar, as it penetrates the cell, forms a glucoside by com- 

 bining with some substance already present, no increase in the number of 



1 De Vries, Jahrb. f. wiss. Bot, 1884, Bd. xiv, p. 589. De Vries himself has corrected his 

 original supposition (Bot. Zeitung, 1879, p. 848) that the organic acids were of primary importance 

 in maintaining turgidity. 



a De Vries, 1. c., p. 581 ; G. Kraus, Stoffwechsel d. Crassulaceen, 1886, p. 4. See also the 

 literature on absorbed soluble substances, Sects. 22 and 109. Hansen (Mitth. a. d. zool. Station 

 zu Neafel, 1893, Bd. XI, p. 258) gives an analysis of the cell-sap of Valonia titricularis. 



3 Pfeffer, Studien z. Energetik, 1892, p. 197. 



