Power to Adjust Parts to Surroundings 229 



therefore, the already-existent tissues are not forcibly bent, 

 but the new tissues grow in such an unequal or differential 

 manner as to swing the parts into their new positions. In these 

 respects phototropic responses are typical of others, for in all 

 cases the power is supplied by the responding plant; and the 

 motor mechanism consists, as a rule, in such differential growth, 

 though occasionally it is of different sort, as we shall presently 

 note. 



Third, the way the light operates in connection with the turning. 

 Since it is not the light but the plant which accomplishes the 

 turning, we still have to seek the nature of the role that light 

 takes in the process. In brief, observation suggests and experi- 

 ment proves that in phototropic responses the plant parts, which 

 in general can grow quite as readily in one direction as another, 

 use the light simply and solely as a convenient guide or signal 

 (called scientifically, but not very fortunately, a stimulus), indica- 

 tive of the most advantageous direction to take. It plays, indeed, 

 very much the same part for the plant that the compass does for 

 the sailor, establishing a definite line of direction, towards, 

 across, or from which, according to circumstances, definite move- 

 ments may be made. This case is typical of the action of stimuli 

 in general; they never take any part in the mechanical accom- 

 plishment of the irritable adjustments, but serve merely as signals 

 for guiding, and sometimes for starting or stopping, the same. 



Fourth, the way the light stimulus is perceived by the plant. 

 The plant has no eyes for the light, as the sailor has for his com- 

 pass, yet it must possess some means of perception of the stimulus 

 else obviously it could not react. The details of the matter are 

 still much in doubt, but in general this much is certain, that the 

 light falling on the sensitive protoplasm of the plant part sets up 

 (probably by chemical means, since the blue rays are mainly 

 concerned) a condition of irritation or strain, which puts the side 

 towards the light in a condition different from the side away 

 from it, and thus establishes the line of light direction. This case 



