90 DR. V. MARTIUS 



chemical effect of light on plants ; because we perceive the former 

 of these exclusively by the aid of an organ, which is wliolly denied 

 to the vegetable world, and even to the lower orders of animals. 



Wliat we call eyes or buds in plants, are undoubtedly more 

 affected by light, as an illuminating power, than any other organs, 

 as far as regards their growth, which I consider, moreover, as a 

 mechanical result. By way of objection to this apparently heterodox 

 view, it will be asked, — Is not light the universal stimulus of 

 plants ? Does it not operate on those green, and blind, deaf, 

 and dumb beings in the same way, as it does on those many- 

 coloured ones among lower animals ? Is not M. Treviranus 

 correct in assuming, that the influence of light among plants 

 supplies that of a nervous system ? Is not the graduated predi- 

 lection of plants for light and shade a proof that they cannot 

 exist without some degree of light, as such ? Is it not the same 

 as regards the indispensable succession of light required by all 

 plants ? And, lastly, have not the various movements of plants a 

 direct dependence on light, apart from heat? I answer in 

 the aflBrmative to all these queries : light, as an illuminating 

 power, is an essential life-stimulus to plants, even though they 

 do not see it ; but only in so far as the plant is, and must be, 

 a moving being. Let us, by way of explaining my meaning, take 

 the homely example of a dark cellar, in which is kept a quantity of 

 fresh potatoes with eyes, and unseemly shaped sprouts. If a 

 pencil of light is thrown among them, so as to touch almost the 

 hinder wall against which the potatoes are piled, when the sun 

 reaches a certain point, the shoots will be seen to strain and 

 elongate themselves in a remarkable degree towards the luminous 

 opening, especially those that are opposite to it. What is the 

 influence which the light exercises in this case, where it does not 

 touch the roots, and cannot therefore operate either by its heating 

 or chemical properties ? It is growth : a sort of organic reacting 

 movement called forth by the undulating motion of the ether 

 caused by light. Thousands of similar observations serve to point 

 at this innate tendency of plants towards light. But this reaction, 

 though proportionate to the inappreciably minute motion of the 

 ether, so that it will probably never be observed in the single 

 cell, is something mechanical, in as far as it affects a body moving 

 in space. It follows hence: — 1, that light, as luminous, belongs 

 not to general stimulants ; 2, that it operates not directly, but 

 indirectly through the atmosphere, or water, or ether; and 

 3, since the calorific and chemical effects of light depend on the 



