2 S. O. MAST 



that referring to the nature of the orienting stimulus. Loeb and 

 some of his followers hold that this stimulus is, in all organisms, 

 animals as well as plants, dependent, in a specific way, upon the 

 amount of stimulating energy received by the sensitive tissue, 

 in accord with the Bunsen-Roscoe law. That is, that a given 

 amount of stimulating energy (which is the product of the in- 

 tensity of the agent and the time it acts) always produces the 

 same effect no matter how these two factors may vary. Thus 

 according to this idea a weak agent acting a long time should 

 cause the same response as a strong one acting a short time. 



This is one of the essential characteristics of a theory of ori- 

 entation which will be referred to as the 'continuous-action' 

 theory. 



Darwin and others maintain that, in some cases at least, the 

 orienting stimulus is dependent upon the time-rate of change of 

 stimulating energy. That is, that if there is no change in such 

 energy there will be no response, no matter how much energy 

 may be received. Response in accord with this idea constitutes 

 the most important feature of a theory of orientation which will 

 be called the 'change-of-intensity' theory. 



Our observations on the process of orientation in Gonium 

 strongly support the latter theory. They also support the con- 

 tention that the eye-spots function as direction eyes essentially 

 as do the eyes in some of the flat-worms. 



The reactions of the colonial organisms have not received much 

 attention, Volvox is the only member of this group that has 

 been extensively studied, and in this form only the responses to 

 light have been thoroughly investigated (Mast '07). It will be 

 expedient to present, in this connection, the essential features of 

 these responses since we desire later to compare them with those 

 observed in Gonium. 



Volvox, like most of the simple green organisms, responds very 

 definitely to light. It orients fairly accurately, and is usually 

 positive in light of moderate intensity and negative in that of 

 high intensity. Orientation is direct, that is, if the position of 

 the source of light is changed after the colonies are oriented they 

 always turn at once toward the light again (provided they are 



