LEAVES WITH BEGARD TO LIGHT. 445 



towards the stem of the plant (fig. 11). According to Sachs's 

 diagram, let o P Q be the stem of the cherry, which bends at the 

 point P so as to make an angle beneath the horizon. Now, if 

 the side Gr of the triangle represent the force of gravity, Sachs 

 assumes that Gr may be resolved into its components Grcos0 

 acting at right angles to the stem P Q, and G sin acting 

 parallel to it. The arrows in the figure are drawn not in the direc- 

 tion in which gravitation acts, but to show in what direction 

 the stem tends to move apogeotropically. Now the force Gr cos 

 acting at right angles to the stem will be constantly acting on 

 different sides of the stem because of the rotation of the klinostat. 

 But the force Gr sin will act all the time parallel to the stem. 

 If is a small angle, Gr sin will be a small force. Consequently 

 the plant will be in the same condition as if it were hung upside 

 down (in the inverse position), only that, instead of the whole 

 force of gravity, only a small part of the force will act on it ; but 

 the result will be a tendency in the leaves to bend backwards 

 towards the stem ; that is to say, this fraction of gravitation will 

 assist epinasty. "When I found out the flexibility of the stem, it 

 occurred to me that the result which I had put down to epinasty 

 might really be due to the weakened force of gravity acting on 

 the leaves. I therefore repeated the experiment with plants whose 

 stems were firmly tied to sticks stuck in the flower-pot, and found 

 the results to be the same. 



This precautionary experiment was in reality hardly needed ; 

 for I had observed the epinastic bending in several cases where 

 there was certainly no flexibility of the stem. 



In the case of large and heavy leaves, such as full-grown cherry- 

 leaves, the influence of their weight on their position must be con- 

 sidered. 



A plant whose leaves were at the following angles, 



i. ii. iii. 



-6° +11° +11°, 



was put in the inverse position, so that the stem hung vertically 

 down ; and now the angles of the leaves were 



i. ii. iii. 



-30° -45° —50°. 



This result is a consequence of the petiole being adapted 

 to withstand the compression of its lower half w T hen the plant 

 is in the normal position ; but the morphologically upper half 



