324 ^Journal of Comparative Neurology and Psychology. 



tactic and crouches down making feeling motions with its head. A very similar 

 condition obtains when it is in water. 



THE REVERSAL OF PHOTOTROPISM BY MEANS OF CHEMICALS. 



A few scattered cases have been known where some substance in solution has 

 changed the light response of an aquatic animal supposedly through its chemical 

 effect. LoEB ('06) alone has attempted a duplication of this process experiment- 

 ally. He has succeeded in making a number of organisms positive by adding 

 a trace of acid to the water containing them. Fresh water Copepods, Daphnia, 

 Gammarus, and Balanus larvae, as well as Volvox, have been made positive 

 in this way. LoEB thinks it probable that the hydrogen ion is the active factor, 

 because normal salts of effective acids produce no change. Alkalies affect the 

 phototropism by destroying the activity of the acid. Volvox differs from the crus- 

 taceans in its behavior toward alkalies in that they act directly upon it to make 

 it positive. Hydrochloric, oxalic, and acetic acids reverse, but less quickly, than 

 carbon dioxide. Low temperature has the same effect as acid upon most of the 

 animals mentioned. It may also be made to reinforce the effect of acid. 



A hypothetical substance within the animals which is affected by the tem- 

 perature and chemical condition of the water is invoked by LoEB to explain these 

 reversals. His theory is also extended to cover changes due to temporary physio- 

 logical states, as for instance the reversal of the Porthesia larva upon becoming well 

 fed. He argues that acid can not favor the production of a chemical compound 

 producing a positive condition because less acid is required to produce the positive 

 state at 10° C. than at 20° while the velocity of a chemical reaction is more rapid at 

 the higher than at the lower temperature. Therefore a substance favoring nega- 

 tive reaction is built up by the protoplasm and its formation or activity is hindered 

 by acid. Or possibly a compound favoring positive reaction and situated in the 

 body may have its activity checked by a different one in the retina. Acids by hinder- 

 ing the formation of this last would produce positive phototropism. An increase 

 in temperature would augment the velocity of its formation and so produce nega- 

 tive response. Loeb did not mention that a rise in temperature makes some ani- 

 mals positive; this fact makes necessary a more general form of the theory. / 



Mast ('07) in his paper on Volvox makes use of the principle of reversible chem- 

 ical reactions in much the same way as Loeb. He further takes into consideration 

 the significance of the substances on the opposite sides of the equation and recog- 

 nizes that any theory must explain reversal in either direction for each kind of stim- 

 ulus affecting the light reaction. He has especially in mind the reversal due to 

 change of light intensity but applies it to other kinds as well. His reasoning is as 

 follows for the case in which great intensity changes the usual positive reaction to 

 negative. Let X stand for a substance, on the one side of the equation and T for 

 one upon the other. Suppose an increase of X beyond a definite amount to produce 

 a positive reaction and of J" a negative one. When X and T are equal the animal is 

 neutral. Since change of temperature produces a new equilibrium in any reversible 

 chemical reaction, thus altering the relative amounts of the substances on opposite 

 sides of the equation, it is reasonable to suppose that light changes can do the same, 

 inasmuch as they also affect the amount of energy involved. If we suppose intense 

 light to fall upon an animal in which the substances are of the proportion X = T 



