446 ME. E. DABWIN ON THE POSITION OF 



of the petiole not being adapted to withstand compression, 

 yields when compressed by the plant being placed in the inverse 

 position. When a cherry-plant is rotating on a klinostat, for 

 instance in the zenith position (as shown in fig. 12), it is clear- 

 that there can be no tendency to curvature in the stem caused by 

 the weight of the parts, since, owing to the rotation, there is no 

 reason why the stem should curve in one direction more than 

 another. 



Fig. 12. 



J2 



1 













A\\ 



V ! 















\1 







B 



k 



k 





M 



A' 







/l 



1 t 



Diagram illustrating the effect of the weight of the leaves. 

 **, axis of klinostat; B, box in which the stem of the plant grows; tt, the 

 transverse plane; /,, a leaf making an angle Gin front ; /,, the position f x 

 ought to occupy when the klinostat has made half a rotation ; f v the posi- 

 tion which it actually occupies, its weight having made it fall forwards. 



The same thing would hold good with the petioles if they had 

 symmetrical powers of resisting weight ; but it has been seen 

 that in the inverse position of the plant the compressing strain 

 comes on the morphologically upper half of the petiole, which is 

 normally subject to tension, and not compression. If, then, a leaf 

 is slightly above the horizon when the plant is in the normal posi- 

 tion, then, when the plant is placed on the klinostat, a leaf hang- 

 ing down, such as/ 1} will retain the original angle which it makes 

 with the transverse plane, since the weight of the leaf is still com- 

 pressing that half of the petiole which is able to bear compression. 

 But when, owing to the rotation, the leaf assumes the position/,, 

 the weight of the leaf will be compressing that half of the petiole 

 which is unable to resist it ; consequently the leaf will fall forward 

 into some such position as/ s . If the leaves were slightly below the 



