LEAVES WITH BEGABD TO LIGHT. 



123 



briurn. These extracts show that Sachs is, on the whole, in 

 favour of an explanation in accordance with De Tries' s views 

 and opposed to that of Frank. There is, however, a feature 

 in Sachs's views which does not occur in De Vries's ; and this is 

 the importance which Sachs attaches to the existence of specific 

 sensitiveness to gravitation and light. 



Fig- 1. 



Fig. 2. 



Diagrams illustrating different degrees of sensitiveness to light and gravitation. 



If AB in fig. 1 represents an organ which places itself so that 

 it is at right angles to the incident light, and if G represents 

 the vertical direction in which gravity acts*, the specific sensi- 

 tiveness to light is such that it requires the light to act with its 

 greatest force, i. e. at right angles, in order to produce aphelio- 

 tropic tendency strong enough to resist and balance the apogeo- 

 tropic tendency j and the specific sensitiveness to gravitation 18 

 such that even when the organ is oblique, and when, therefore, 

 the stimulus of gravitation must be feeble, the amount of apogeo- 

 tropic action called into play is enough to balance the apheho- 

 tropism produced by the light. 



The or-an CD, shown in fig. 2, assumes a horizontal mstead of 

 an oblique position, because its specific sensitiveness to light and 

 gravitation are different to those of AB : it is more sensitive to 

 lUt and less so to gravitation ; so that an amount of apheho- 

 tropism sufficient to balance the apogeotropism is produced by 

 the weakened stimulus of oblique rays of light, and the necessary 

 amount of apogeotropism is not produced until the organ is in 



* The barbs of the arrows are made to represent the directions in which 

 light and gravitation tend to cause the organ to bend ; thestems of the arrows 

 give the angles at which light and gravitation act. 



