I2 PHYSICS OF STREAMING 



hand the direct action of a rise of temperature is to increase the respiratory 

 activity, and to decrease the viscosity of the moving fluids ; both of which 

 factors are more powerful ones than the indirect action of the increased 

 osmotic pressure. 



4p 



SECTION 4. Percentage of Water. 



Velten, and subsequently Hauptfleisch \ concluded that there is 

 a certain optimal percentage of water, varying in different objects, at which 

 streaming is most active. Embryonic cells containing relatively little 

 water show no streaming, which commences as they grow older, becomes 

 active circulation when vacuoles appear, and turns into rotation when 

 a single large central vacuole is formed. The presence of a vacuole, 

 however, simply shows that the percentage of water in the cell increases, 

 and says nothing as to the percentage present in the protoplasm, which 

 is the really important point. The latter is dependent upon the osmotic 

 pressure of the cell-sap, the force of imbibition, and upon the amount and 

 character of the soluble osmotic materials present in the protoplasm. 



Little is known as to the relative percentages of water in the proto- 

 plasm of young solid cells, and of older vacuolated ones. In the former 

 case the osmotic pressure of the protoplasm and of the entire cell are the 

 same. During active growth this pressure can never be considerable, since 

 the consumption of food materials is rapid, and the cell- wall thin and ex- 

 tensible. The latter appears in fact usually to lie between 3 to 6 atmospheres, 

 which is about the minimum osmotic pressure (3 to 5 atms.) to which adult 

 phanerogamic cells can be reduced by starvation. The values obtained 

 by applying plasmolytic solutions to growing apices or apical cells are 

 frequently much higher than the true ones, since the highly extensible 

 cell-walls of such cells may collapse to a considerable extent before the 

 protoplasm shrinks away from them. Indeed, in certain cases, the cells 

 cannot be plasmolyzed at all 2 . If, however, the osmotic concentration 

 is observed at which the cell begins to decrease in size, this gives its true 

 osmotic pressure. Actively growing root apices, the meristems of seedlings, 

 the apical cells of Selaginella and ferns, gave approximately the values 

 already mentioned, but any retardation of growth is rapidly followed by 

 a rise of osmotic pressure at the growing points if the plant is well 

 nourished. If solutions of glycerine are applied to apices of Selaginella, 

 they must be fairly strong to produce any distinct retraction. Thus 

 a 10 per cent, solution may produce no retraction, although its osmotic 

 pressure is 30 atmospheres, and 10 to 20 per cent, solutions which cause 

 distinct plasmolysis of the uncuticularized cells behind the apex may not 



1 Velten, Bot. Zeitg., 1872 ; Hauptfleisch, Jahrb. f. wise. Bot., 1892, Bd. xxiv, p. 213. 



2 Cf. Pfeffer, Druck- n. Arbeitsleistungen, 1893, p. 307. 



