196 LECTURE XII. 



Indeed, it is not going too far to say that every dependence of a physio- 

 logical function upon any one external influence assumes the form of a curve, 

 first ascending and then" again descending; and that we have here one of the 

 fundamental laws of physiology. Since now we may term each dependence of 

 a vegetadve phenomenon upon external influences irritability, the form of curve 

 mentioned represents the fundamental law of irritability. This, expressed in words, 

 would run thus ; — If the intensity of an external influence {i. e. of a stimulus) increases 

 more and more, the effect of the stimulus, or the corresponding function of the 

 plant also rises, but only up to a certain degree, the optimum of the former; if 

 it exceeds this optimum, the effect on the plant is diminished, until at a certain 

 most intense influence the functional capability of the plant ceases. And further : 

 as the influence of the temperature only stimulates the plant when it attains 

 a certain height, so also every other external force or stimulus only then appears 

 to exert a perceptible effect when it has attained a certain intensity, sufficient to 

 overcome the resistances existing in the plant. 



The law of the dependence of the phenomena of vegetation upon external 

 influences, which I have here attempted to make clear, gives, as, is ever the case 

 with natural laws, the relations between cause and effect in the most general 

 and therefore abstract form possible. Thus, even the simply formulated law of 

 gravitation is only a quite general abstract formula for an endless variety of 

 events. The path described by a stone thrown into the air, as weU as the 

 arrangement of the planetary system ; the apparent irregularity of the movement 

 of comets, as well as the flowing of water from the continents towards the ocean, 

 and the ebb and flow of the latter, are ruled by the abstract proposition that 

 bodies attract one another in proportion to their mass, and in inverse proportion 

 to the square of their distances : or, to select yet another example, the endless 

 variety of natural phenomena produced by the reflection of rays of light ; the 

 ordinary reflected image of our own person seen in the mirror, the focus of 

 a concave mirror, the Fata IMorgana, and the signals of the Heliograph, all 

 come under the abstract natural law that rays of light are reflected from the 

 surface of a body at the same angle as that at which they meet it. Just as 

 little as we regard the fact that this geometrical expression underlies this endless 

 variety of figures and phenomena, so litUe do we regard the above-described 

 curves as representing in endless variety the relations between plants and the 

 external world. Since it is the object of this lecture to give an account as clear 

 and general as possible, and not merely abstract but also concrete, we will now 

 pause again at a sketch, though slight, of those phenomena of vegetation which 

 illustrate the dependence of the life of the plant upon external influences in a 

 particularly impressive and intelligible form. 



collected. Pfeffer gave further investigations, made in my laboratory, in Arbeiten des bot. Inst, 

 in Wzbg., B. I. p. i. I referred to the errors of other observers on this question in the same (B. I. 

 p. 276), in the treatise 'Die Pflanze und das Auge als verschiedene Reagentien für das Lichte Here 

 I have still to remark that the expression for the dependence of the evolution of oxygen upon 

 coloured ligiit — namely, a dependence upon the wave-length — correctly employed in the text, was 

 first established by me in the third edition of my 'Text Book' (1873) and in the fourth edition 

 (1874), p. 718, which Pfeffer has not quoted in his 'Pflanzenphysiologie,'' p. 21 j. 



