32 The Living Plant 



with a view to finding an explanatory principle : then to express the 

 most probable conclusion in tentative form as an hypothesis: 

 and finally to devise experiments whereby the truth or falsity 

 of the hypothesis may be tested; these are the constituents of 

 that scientific method through which all of our great scientific 

 triumphs have been won. Hypothesis is a kind of a scout which 

 Science sends on ahead to spy out the way for a further advance.* 

 For the completion of our subject of photosynthesis, there re- 

 mains but one matter of consequence, and that is the explanation 

 of the association of light and chlorophyll with the process. We 

 have seen earlier that the chlorophyll occupies a position between 

 the light and the new-made starch or sugar, which fact implies 

 that it forms a necessary link between the two. This in turn 

 would suggest that the chlorophyll perhaps acts on the light in a 

 way to make it available for the photosynthetic process. Tak- 

 ing this hypothesis for guidance, we turn to investigate the effect 

 that chlorophyll exerts upon the light which penetrates into it. 

 Now the sunlight, as everybody knows, is a composite mixture 

 of vari-colored lights, which, taken together, give the impression 

 of whiteness. If this sunlight, however, be passed through 

 chlorophyll, whether a living leaf or a solution in alcohol, there 

 issues, as the reader will recall, only a clear green, or yellowish- 

 green, light; and this fact seems to imply that all of the colors 



* That this is in practice, as it is in theory, the method of scientific men in their 

 researches is illustrated by the following passage from the writings of the great 

 German physiologist, Sachs. In connection with this very subject of starch forma- 

 tion, he tells of his preliminary observations, on the basis of which, he says, " I 

 came to the conclusion in 1862 that the enclosed starch, which had already been ob- 

 served in the chlorophyll-corpuscles by Naegeli and Mohl, is to be regarded as the 

 first evident product of assimilation [i. e., photosynthesis] formed by the decom- 

 position of carbon dioxide. I said to myself, if this view is right, the formation of 

 starch in the chlorophyll-corpuscles must cease on the exclusion of light, since the 

 decomposition of carbon dioxide can then no longer take place; and that in like man- 

 ner renewed access of light to the chlorophyll-corpuscles must also bring about a 

 renewal of the formation of starch in them. These and similar deductions were con- 

 firmed by appropriate investigations." (Lectures on the Physiology of Plants, 

 Oxford, 1887, p. 307.) 



