134 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



The incrustations of ferric oxide, formed on many algae, bacteria, &c., present 

 us with similar problems as to the causes of their origin l . In certain cases, the 

 oxidation of ferrous salts to ferric oxide may render energy available for metabolism 

 (Sects. 63 and 96). 



SECTION 24. Osmotic Pressure in the Cell. 



The non-diosmosing substances dissolved in the cell-sap exert in every 

 case an osmotic pressure determined by definite physical laws, and this in 

 ordinary turgescent cells usually averages from five to ten atmospheres. 

 By this pressure, directed in the first instance against the semi-permeable 

 vacuolar membrane, the viscous protoplasm, which always forms a complete 

 and continuous enclosing envelope, is pressed against and brought into the 

 closest contact with the cell-wall (Fig. 10). The cell-wall reacts corre- 

 spondingly with an equal but internally directed pressure. The intensity 



of the osmotic pressure and 

 the degree of cohesion of the 

 cell-wall determine to what 

 extent the latter will be 

 stretched or whether it will 

 be ruptured as happens, 

 for example, when various 

 pollen grains are placed 

 in pure water 2 . It is by 

 * means of this tension, to 



FlO. 10. From the root of Zta Mais, (a) in water ; (*) plasmolysed in i U ^u r .. 



5 per cent. KNO 3 ; (c) slight plasmolysis in 2-3 per cent. KNOs. WnlCh the name OI ttirgOT 



is given 3 , that thin-walled 



cells are stretched and stiffened, just as a bladder or india-rubber balloon 

 stiffens when forcibly injected with water or air 4 . Hence plants collapse and 

 wither when by plasmolysis or by transpiration so much water is removed 

 from the cells that the osmotic tension and the turgid condition connected 

 therewith are partly or entirely removed. 



Without the resistance offered to extension by the cell-wall, no 

 marked osmotic pressure could be attained, for naked vacuolated masses 

 of protoplasm are stretched and ultimately ruptured by a relatively trifling 

 internal pressure. Fluid lamellae, however, when pressed against a rigid 



1 Hanstein, Sitzungsb. d. Niederrh. Ges., May, 1878 ; Winogradsky, Bot. Zeitung, 1888, p. 261 

 Mohsch, Die Pflanze u. ihre Beziehung zum Eisen, 1892, pp. 18, 30, 60. 



' See Lidforss, Jahrb. f. wiss. Bot., 1896, Bd. xxix, p. i. 



' This tension is for the most part due to osmotic energy, and hence it is admissible to designate 

 the total hydrostatic tension, regardless of its origin, as turgor.' Cf. Pfeffer, Plasmahaut u. 

 Vacuolen, 1890, p. 297. 



Cf. Nageli nnd Schwendener, Mikroskop, 1877, 2. Aufl., p. 404. 



