CHAPTER XVII 



EFFECT OF LIGHT UPON GROWTH 



We have seen in Chapter VII that light profoundly affects 

 metabolism, not only by its heat rays, which are essential to 

 the process of starch formation in green plants, but also by its 

 more chemically active rays, which produce chemical changes 

 in organisms of all classes. We have now to see how, as a 

 result of these effects, light has an influence upon growth. 



§ 1. Effect of Light on the Rate of Growth 



Two fundamental principles, established in the First Part of 

 this work, must be recognized at the outset of this discussion, 

 or else the data which have been accumulated will appear con- 

 fused and meaningless. The first principle is that white light 

 is not a constant, definitely determined thing, but varies in its 

 intensity, and, as we saw in the case of phototaxis (I, p. 201), 

 at the different intensities produces diverse, even opposing, 

 effects. The second principle is that not all organisms are sim- 

 ilarly affected by the same intensity of white light. This is 

 because they are attuned to diverse intensities of light, so that 

 the same intensity will call forth a dissimilar response in differ- 

 ent organisms (I, p. 196). It is a consequence of these two 

 principles that, when we classify our data on the basis of the 

 intensity of the light, we shall find dissimilar effects in each 

 class ; or when, on the other hand, we classify on the basis of 

 results, we shall have to consider in each class apparently 

 diverse causes. Since, however, results are always more cer- 

 tain than causes, we adopt the method of treatment on the 

 basis of results. 



1. Retarding Effect of Light. — We have already in the 

 First Part of this work seen that Protista are injuriously 

 affected by strong sunlight, cultures of bacteria becoming 



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