IRRITABILITY 205 



with the action of gravity. Czapek's conclusion is sup- 

 ported by a study of grass-haulms.* 



When the root-tip is stimulated by the action of gravity a 

 bending takes place further back, in the region which is most 

 rapidly elongating. This region, called the motor zone, can 

 bend only if there are differences in the rate of growth or 

 in the tissue tensions in its different parts. It is not neces- 

 sarily the case that all roots behave alike in the means 

 any more than in the manner of curvature, although it 

 seems probable that in roots of similar structure the me- 

 chanics of curvature will be similar. This may account in 

 part for the diverse views regarding the mechanics of geo- 

 tropic curvature. Thus, according to Ciesielski, f the cells of 

 the lower (concave) side of the root are forcibly com- 

 pressed, even wrinkled, by the more than average growth of 

 the cells of the upper ( convex ) side. Sachs I denies that the 

 cells of the upper side always grow faster, and attributes 

 the pronounced bending to the diminished growth rate of 

 the cells of the lower side. MacDougal,§ employing the 

 imbedding methods now in general use but unknown to 

 Sachs when he wrote on this subject, comes to essentially 

 the same conclusions as the great master in plant physi- 

 ology more than twenty-five years before. Pollock,! study- 

 ing curvatures induced by injury (traumatropic) instead of 

 by gravity (geotropic), concludes that the mechanism of 

 root-curvature consists in changed tissue-tensions in the 

 stimulated roots, the normal tension between cortical paren- 

 chyma and axial cylinder increasing on the upper ( convex ) 

 side and decreasing on the lower (concave) side. Though 

 this last view may be correct as regards some if not all 

 roots, it must be conceded that growth makes permanent 



* Pertz, Miss D. F. M. On the gravitation stimulus in relation to posi- 

 tion. Annals of Bot., XIII., p. 620, 1899. 



t Ciesielski, Th. Untersuchungen iiber die Abwartskriimmung der Wur- 

 zel. Cohn's Beitr. zi. Biol. d. Pflanzen, Bd. I., 1871. 



J Sachs, J. von. Gesammelte Abhandlungen, Bd. II., p. 852. 



§ MacDougal, D. T. The Curvature of Roots. Bot. Gazette, vol. 23, 

 1897. 



If Pollock, J. B. Mechanism of root-curvature. Bot. Gazette, vol. 29, 

 1900. 



