Mast, Light Reactions in Loivcr Organisms. 159 



No work, as far as I know, has been done directly on the effect 

 of change in light intensity on equilibrium in chemical reaction; 

 but we know^ that light does affect many chemical reactions, 

 and since we must regard chemical reaction in general as rever- 

 sible, it seems reasonable to assume that the relative amount 

 of different substances present in a mixture is dependent upon the 

 li2;ht intensity, provided the chemical reaction between the sub- 

 stance is at all affected by light. This means that substances 

 in chemical equilibrium in one light intensity will not be in equi- 

 librium in another, so that the direction in which the reaction takes 

 place faster depends upon the light intensity. 



To explain reversal in the sense of reaction on the basis of 

 chemical reactions induced by light let us assume: (i) That Vol- 

 vox contains substances X and T, the chemical reaction between 

 which is regulated by the intensity of light; (2) that a sub-optimum 

 intensity favors the formation of substances represented by X and 

 a supra-optimum those represented by T; and (3) that the colonies 

 are neutral in reaction when there are T substances in one member 

 of the equation and X in the o^^her; positive when one member con- 

 tains {X +) substances and the other (T -), and negative when 

 one contains (X -) and the other (T + ). Can the change in sense 

 of reaction as represented in paths J and B, Fig. 14, p. 156, be 

 explained on the basis of these assumptions ^ 



The colony which produced path J was positive when put into 

 the aquarium at n. In accordance with our assumption it, there- 

 fore, contained (X +) and (T -) substances. The intensity at 

 n was relatively low so that the chemical reaction favored the 

 formation of compounds represented by X. This may be expressed 

 thus (X +) ^(T -), indicating that the reaction toward X takes 

 place faster than that toward T. The increase in the X and 

 decrease in the Y substances continued until a state of equilibrium 

 was attained or the organism reached ?/ and c, where the light 

 from the glower was turned off and that from the arc turned on, 

 and the colony was thus exposed to light of supra-optimum inten- 

 sity. Why did it not then turn from the source of light at once .'' 

 According to our assumption, because it contained (X +) and 

 (J* -) substances. But since the colony was in a supra-optimum 

 intensity, the chemical reaction favored the formation of T sub- 

 stances at the expense of X, represented thus (X +) ^ (^ ")• 

 As soon as this reaction had continued far enough so that {X + ) 



