TUEGIDITY 89 



d) Curvature. — Since most plant curvatures are due to 

 modified growth, it is to be expected from what has just 

 been said that turgor must play an important part in these 

 phenomena. A discussion of the relations of this question 

 will be found in Pollock's paper ^ on root curvature. To 

 enter into this much mooted question would be going too far 

 afield from the present subject. 



Other more rapid curvatures (such as those of the pulvini 

 of the leaves of the Mimosa, the stamens of Berberis, various 

 tendrils, etc.), which are not due to growth, have already 

 been mentioned. They owe their existence entirely to turgor 

 changes. 



e) Work. — Turgor is also of great benefit to the plant 

 in that it gives it a means of doing work, of overcoming 

 resistance. The lifting of sidewalks and buildings and the 

 splitting of cliffs by the roots of trees are examples of the 

 extent to which this pressure may be developed. For it must 

 be remembered that the growing region of any plant is 

 always mechanically weak; it owes practically all its power 

 of resistance to the turgidity of the cells. Pf effer ' has made 

 many tests and measurements in this field, and Rodewald' 

 has applied mathematics to the problem, showing how 

 osmotic pressure is a very considerable source of energy to 

 the plant. Of course the energy ultimately comes from the 

 heat of external objects. 



VIII. SUMMARY OF THE OHAPTEE 



Turgidity is the immediate result of osmotic pressure within 

 the cell. It arises from pressure developed within the cell 

 sap of solutes which are unable to penetrate the surrounding 



1 J. B.' Pollock, "The Mechanism of Hoot Curvature," Bot. Gaz., Vol. XXIX 

 (1900), pp. 1-63. 



2 W. Pfeitee, " Druck und Arbeitsleistung durch wachsende Pflanzen," Abhandl. 

 d. k. Sachs. Bes, d. Wiss. zu Leipzig, math.-physik. Klasse, Vol. XX (1893), p. 235. 



3H. EoDEWALD, "Ueber die durch osmotische Vorgange mOgliche Arbeitsleistung 

 der Pflanzen," Ber. d. deutsch. bot. Ges., Vol. X (1892), pp. 83-93. 



