f 



228 The Living Plant 



be found in an advantageous functional adjustment of those 

 parts in relation to the direction of light. And this principle of 

 advantageous individual adjustment of parts is characteristic of 

 irritable adjustments in general. 



Second, the mechanical method whereby the turning is effected. 

 The turning of the leaves, stems, and roots 

 into their respective new positions requires 

 both a considerable power and a definite 

 mechanism. Now it is quite evident that in 

 phototropism neither of these is supplied by 

 the light, for that has no power at all to lay 

 bodily hold on the parts and forcibly pull, 

 bend, or push them into their respective 

 positions, while it is easy to prove on the 

 contrary that the power is supplied by the 

 plant, and derived from its own respiration. 

 Thus, if oxygen be withdrawn from a cham- 

 ber in which a symmetrical plant is sub- 

 jected to one-sided light, not a trace of 

 phototropic response ever follows. The con- 

 nection will be clear to the reader: The 

 FIG. 77.-Suecessive stages response requires energy, energy depends on 

 in the downward turning respiration, respiration demands oxygen ; 



of a root, showing, by the 



spread of the marks, that therefore no oxygen, no response. And as 

 L he e/ect P e a d re by nTTftS to the mechanism of the turning, that also 

 entiai growth and not by - ft determined by experiment, for if 



the bending of tissues al- ? 



ready formed. The tn- stems, petioles, or roots are marked across 



angular piece is a paper j v i_ r ji i . 



index. (Copied from with evenly-spaced lines before the plant is 

 exposed to one-sided light, then the marks 

 spread apart in a way to prove that the bending accompanies 

 growth in those parts, and is due to a more rapid growth on 

 one side than the other, on just that side, indeed, where it is 

 requisite in order to swing the parts concerned into the advan- 

 tageous positions (figure 77). In phototropic adjustments, 



