140 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



glanaim, &c., attain their power of growing on concentrated solutions, for 

 they are able to remain turgid, even when the osmotic concentration of the 

 cell-sap is equivalent to thirty-eight per cent, of NaNO 3 ( = forty-five per 

 cent, of KNO 3 = 157 atmospheres) 1 . Certain powers of accommodation are 

 exhibited by other plants also, but only to a relatively trifling extent. 

 Since the normal turgid pressure in the cells of marine algae is in general 

 similar to that in terrestrial plants, it follows that in comparison with the 

 latter the osmotic value of the cell-sap must be higher by about three 

 per cent, of Na Cl (3-7 per cent, of KNO 3 ) 2 . 



It is hardly surprising that, if plants which have become accommo- 

 dated to a concentrated solution are suddenly placed in pure water, the 

 increased internal pressure may be so great as to cause their cells to burst, 

 for, in the case of Aspergilhis, this pressure may amount to as much as 

 160 atmospheres. The strength of the cell-wall develops correspondingly 

 to the demands made upon it, and the small size and diameter of the cell 

 aid markedly in the attainment of the necessary resistant powers, for these 

 increase as the radius decreases 3 . Many special features still require 

 explanation, as, for example, how a bivalved diatom retains its form in 

 spite of the internal hydrostatic pressure 4 . It is clear, moreover, that the 

 turgiclity produced by the storage of soluble materials will depend upon 

 whether these are osmotically active or not. 



The actual turgidity of the cell is due to the sum of the osmotic 

 pressures exerted by the different substances present in the cell-sap. Hence 

 it may happen that in one cell fifty to seventy per cent, of the osmotic 

 energy is due to a substance, which is entirely absent from another cell. 

 From the table on p. 146 it can be seen that the hydrostatic pressure 

 is usually equivalent to that of solutions of NaCl, KNO 3 concentration of 

 from one to five per cent., or to that of similar solutions of the organic 

 acids (malic, tartaric, citric, oxalic) or their salts, which are so widely 

 distributed in plants ; whereas three to five times as much cane-sugar 

 would need to be present to produce the same result ; and in a cell filled 

 with a thick solution of gum-arabic, or other colloid substance, the osmotic 

 pressure would perhaps not be greater than that of a one per cent, solution 

 ofKNO 3 . 



The fluid obtained by pressure gives a fairly accurate representation 

 of the composition of the cell-sap, especially when the protoplast is 



1 Eschenhagen, Einfiuss versch. Concent, auf Schimmelpilze, Leipziger Diss., 1889. Further 

 literature : K. Bruhne, Beitr. z. Physiol. u. Morph. cl. nied. Organismen von Zopf, 1894, Heft 4, p. i 

 (Fungi;; B. Stange, Bot. Zeitung, 1892, p. 277 (Phanerogams); A. Richter, Flora, 1892, p. 9 

 (Algae) ; A. Fischer, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 151 (Bacteria). See Sect. 73. 



2 See also Oltmanns, Sitzungsb. d. Berl. Akad., 1891, x, p. 201. 



3 PfeRer, Die period. Bewegungen der Blattorgane, 1875, p. 114; Nageli und Schwendener, 

 Mikroskop, 1877, 2. Aull., p. 412. 



4 See O. Miiller, Ber. d. Bot. Ges., 1889, p. 74. 



