68o THE POPULAR SCIENCE MONTHLY. 



analogous in the molecular changes, and need not be considered 

 apart. The mechanism is diJBEerent, though the forces are the 

 same ; just as steam is the same, whether it runs a rolling-mill or 

 a locomotive. 



The chemical or molecular theory of light-action is greatly 

 strengthened by two considerations : The first is, that a latent pe- 

 riod, after stimulus and before reply, can be detected in almost 

 every case. This, as evidence of chemical action, is conclusive. 

 Second, the researches of Wiesner, and especially those of C. M. 

 Guillemin, recorded in the "Annales des Sciences Naturelles," 

 series iv, vol. vii, in which careful investigations are made into 

 the kind of light which has the greatest effect in heliotropism, 

 must be noted. With regard to assimilation — as we should indeed 

 expect — the yellow rays are found to be the most favorable ; but 

 when light is studied with reference to irritability, the results are 

 entirely different. The maxima of action are now found to be at 

 the extreme ends of the spectrum ; the ultra-red and the ultra- 

 violet or so-called actinic rays seem to be the ones best capable of 

 giving the necessary stimulus. The minimum is found to be in 

 the blue, near the " F " line. That this " division of labor," as it 

 were, among the light-rays, should be so evident and so constant, 

 offers strong testimony in favor of the theory that irritability 

 and assimilation are equally molecular in their nature (if we may 

 use such an expression), and the whole hypothesis seems unusu- 

 ally clear and satisfactory. 



To return for a moment, now, to the potato-shoot grown in 

 darkness, one other peculiarity besides those mentioned might 

 have been noticed. That was this : the angle between leaves and 

 leaf -axis, or stem, was always more acute than in the normally 

 grown plant. The whole etiolated shoot seemed to be straining 

 toward the light. Kraus believed that this was due to imperfect 

 anatomical development in the fibro-vascular bundles ; but such 

 a view is scarcely confirmed by the facts, for etiolation is con- 

 cerned not with the fibrous system alone, but with the fundament- 

 al. Rauwenhoff supposed the vertical position of shoots grown 

 in darkness attributable to absence of heliotropism, and the con- 

 sequent unmodified action of another force in plant-physiology, 

 namely, negative geotropism, or, more clearly, " negative gravity." 

 This is a kind of irritability, in view of which plant-shoots tend 

 to increase in length in a direction opposite to the terrestrial at- 

 traction. This, he thought, might be favored by the feeble thick- 

 ening of the cellular tissues. In this Sachs is inclined to support 

 Rauwenhoff ; but it seems probable that the whole matter will 

 have to be laid at the door of heredity. The plant which has al- 

 ways struggled upward to the light will continue to do so when 

 placed in darkness, and all its efforts will be concentrated upon 



