120 TROPISMS 



culty arose in regard to the action of gravity upon orien- 

 tation, since it was contrary to the definition of a "stimu- 

 lus" that the mere permanent lying in a horizontal posi- 

 tion should cause the apex of a stem to bend upward. 



All these difficulties disappear if we take the law 

 of chemical mass action into consideration. Light acts 

 not as a "stimulus" but acts by increasing the mass 

 of certain chemical compounds, and it is the mass of 

 these products which is responsible for the effect of light. 

 Now, mass action is not proportional to the rapidity 

 of the change of acting masses but to the acting mass 

 itself. When two sides of an organism are struck by 

 light of different intensity the quantity of photochemical 

 products on both sides becomes unequal. In galvano- 

 tropism the galvanic current alters the distribution of the 

 mass of certain ions along the nerve elements. 



It can be shown that gravitation acts by influencing 

 the distribution of chemical substances in an organism. 

 When the stem of a plant is put into a horizontal position 

 certain chemical substances gather in greater concen- 

 tration on the lower side of the stem; and this causes a 

 difference in the velocity of chemical reactions between 

 the lower and the upper side. As a result of this we 

 notice the bending. In the normal upright position of 

 the plant the same substances were distributed equally 

 about the axis of symmetry. 



The following facts may be offered as a proof for this 

 statement. 526 When w T e put a piece of the stem of Bryo- 

 phyllum calycinum in a horizontal position it soon bends 

 and gradually assumes the form of a U with the concave 

 side above (Fig. 35). This bending is due to the fact that 

 the cortex on the under side of the stem grows in length 

 while the cortex on the upper side remains unaltered 



