1899.] NATURAL SCIENCES OF PHILADELPHIA. 223 



believe that the transference of liquid from cell to cell, resulting in 

 the alternate rigidity of the upper and lower portions of the 

 petiole, has a very considerable bearing upon the resulting move- 

 ments. The movements are due to the gradual passage of sap 

 through the contractile protoplasmic sac of each cell into the inter- 

 cellular spaces, or they in all probability are due to the movement 

 of the liquid from cell to cell by means of the protoplasmic 

 bridges, so that one part of the leaf becomes highly turgescent and 

 the other part more or less flaccid. Cold weathei*, therefore, sets 

 the liquids in motion toward the upper side of the petiole and leaf. 

 The result of this motion of sap would be the downward flexure of 

 the leaf-stalk and the inward rolling of the leaf. When any 

 branch with hanging leaves is brought into a heated room the 

 liquid is again conveyed to the cells lying near the lower surface 

 and the blade and petiole right themselves. That there is some 

 movement of cell-sap is evident on watching the change of color 

 of the leaves after they are brought indoors. In the cold they 

 are of a blackish green color, but on full expansion they assume 

 a brighter green, which becomes lighter as the temperature of the 

 surrounding air rises. 



Turgidity is then the main factor in the mechanism of these 

 movements ; its mechanical importance is further strikingly illus- 

 trated by the great rigidity of the turgid members, and by the great 

 force, equivalent in parts of some plants to twenty times the atmos- 

 pheric pressure, which they develop in ojjposition to external 

 resistance, as when the roots of trees cause the splitting of walls 

 or pavements. Although one essential factor in turgidity is the 

 purely physical osmotic activity of substances in the cell-sap, it 

 must not be forgotten that it also depends upon the resistance 

 offered by the protoplasm to filtration under pressure ; so that the 

 maintenance of turgidity is after all a vital act. The maintenance 

 of turgidity appears, in fact, to depend upon a certain state of 

 molecular aggregation of the protoplasm lining the cell- wall, in 

 which it offers resistance to the escape of the cell sap; whereas in 

 the flaccid condition the state of molecular aggregation of the 

 protoplasm is such that it readily permits the escape of cell-sap, 

 under the elastic pressure of the cell-wall, either into the inter- 

 cellular spaces or through the protoplasmic bridges into adjoining 

 cells, which thus become more turgid. 



