444 



MB. F. DAHWIN OS THE POSITION OF 



this instance to keep the leaf in its previous position without 

 the aid of apogeotropism, which in all the other experiments seemed 

 to be needed as an antagonist to epinasty. 



Sources of Error. — Before summing up the results of the expe- 

 riments with cherries, it will be well to point out a source of error, 

 which, however, does not vitiate the results given. On June 9th, 

 after a cherry-plant had been rotating in the zenith position during 

 my absence from home, since May 2Gth, it was noticed for the 

 first time that the stem, which had grown much in my absence, was 

 flexible, and bent with the weight of the leaves, so that it did not 

 remain horizontal, but was constantly inclined at from 10° to 15° 

 beneath the horizon in spite of the rotation of the klinostat. Thus 

 the stem was always in the position shown in fig. 10, and conse- 

 quently the leaves 1 1, arising from a flexible part of the stem, 

 tended to bend apogeotropically towards the stem. If we make 

 use of Sachs's method of estimating the strength of the geotropic 

 stimulus as in some way proportional to the angle which the 

 organ acted on makes with the line of gravity *, we shall see that 

 the result of the flexibility of the stem is a small constant apo- 

 geotropic stimulus, tending to make the leaves bend backwards 



Diagrams illustrating the result of the flexibility of the stem. 

 Fig. 10. 

 I. 



Jck, axis of klinostat; B, box in which plant grow?; oPQ, the flexible stem 

 making an angle below the horizon. 



Kg- 11. 



G cos & \« 



* *, axis of klinostat ; B, box in which plant grows ; o P Q, stem bent at P »o 

 as to make an angle below the horizon ; G, the hypothenuse of the triangle, 

 represents the force of gravity ; G cos and G sin the resolved parts of 

 G perpendic ular and parallel to the stem P Q. 



* 'Arbeiten,' Bd. ii. p. 240. 



