230 The Living Plant 



is typical of all stimuli, which act by producing in the sensitive 

 protoplasm on which they impinge a condition of differential 

 irritation or strain which serves to impress a line of direction on 

 the part concerned. Then the part is swung by the motor mech- 

 anism into a position where this condition of strain is the same 

 all around, which position is kept in the subsequent growth. Ob- 

 viously only those agencies can act as stimuli at all which can 

 thus produce a differential state of the protoplasm, and con- 

 versely, any agency capable of producing such a condition can, 

 theoretically, act as a stimulus. And as to how strong a stimulus 

 must be to produce an effect, it is only essential that it have 

 enough power to produce the impression of differential strain 

 on the sensitive protoplasm; and above that degree its strength 

 does not much matter. 



Fifth, how it is that a single uniformly-acting stimulus can evoke 

 different directions of turning. The fact that in phototropism the 

 light neither pushes nor pulls the parts to their positions, but acts 

 simply as a guide to direction, involves the corollary which is 

 confirmed by experience, that it is exactly as easy for parts of the 

 plant to grow away from or across the light as towards it, pre- 

 cisely as the sailor, guided by his compass, which neither pushes 

 nor pulls him over the sea, can steer as easily to the south, east, 

 or west as to the north where it points; and the reader should 

 learn to think of all stimuli in this way. But if the parts of the 

 plant can turn as easily in one direction as another in relation to 

 light, what feature of their growth-mechanism is it which sends 

 stems so unerringly towards it, leaves across it, and roots from 

 it? Here again there is very great doubt as to particulars, but 

 hardly any as to principle, which can thus be illustrated. In a 

 locomotive, as most people understand, there is a certain lever, 

 which when set in one direction determines that the engine shall 

 move forward, and when set in another, that it shall move back- 

 ward, after the steam is turned on; and an engine is easily imagi- 

 nable in which, with the lever in yet a third position, the move- 



