SPECIFIC ENERGY OF IRRITABLE ORGANS. 6oi 



a rule their irritable movement results from gentle contact with a solid bodj^, but 

 vibration as well as cutting off or burning the apex of the tendril causes its upper side 

 to curve, since it grows more rapidly than the lower side, and these curvatures only 

 differ in the different cases because external mechanical conditions influence the 

 course of curvature. The identity of the effect of stimulation in cases where totally 

 different stimuli act on the growing root-tips is particularly striking : illumination from 

 one side, geotropic action, pressure on a solid body, the influence of adjacent moist 

 surfaces, and, as it appears from Elfving's recent researches, even constant electric 

 currents produce exactly the same kinds of curvatures. The organ possesses only 

 this one mode of responding to stimuli of the most various kinds. In our subsequent 

 consideration of the phenomena of irritability in Mimosse, when the details are 

 more closely examined, we shall moreover be permitted to obtain a deeper insight 

 into the specific energy, since we shall there be in a position to understand the 

 mechanism of stimul9,tion generally, and therefore also to comprehend why entirely 

 different stimuli must call forth the same effect in the organ. 



In concluding these considerations there are still a few remarks to be made on 

 the significance of irritability for the life of the plant generally. Clothed in words 

 somewhat different 'from those employed at the beginning of the present lecture, 

 we may define irritability as the mode in which a given organism, or a definite 

 organ belonging to it, reacts to the external world. On this continuous reaction 

 between the external world and organic structure depends generally the life not 

 only of plants, but of animals also, as was described in detail in Lecture XII. 

 The organism itself is only the machine, consisting of various parts, and which 

 must be set in motion by the action of external forces : it depends upon its structure 

 what effects these external forces produce in it. It would betray a very low 

 level of scientific culture to see in this comparison a degradation of the organism, 

 since in a machine, although only constructed by human hands, there lies the result 

 of the most profound and careful thought and high intelligence, so far as its struc- 

 ture is concerned, and in it there subsequently become effective the same forces of 

 Nature wluch in other combinations constitute the vital forces of an organ. The 

 comparison of organic life with inorganic processes can thus only be held as a 

 debasement of the former, if one has become so foolish as to look upon the latter as 

 something low and common, whereas the incomprehensible magnitude and all-per- 

 vading power of Nature is equally evident in both cases. 



If then irritability is, as said, fundamentally nothing other than the reaction of 

 the organs towards the outside wqrld, determined by means of their structure, 

 which in fact argues by itself a similarity in kind of both, it follows thence directly 

 that the phenomena of irritability both in the vegetable and animal kingdom must in 

 the main be full of purpose. What I understand by this has already been stated at 

 the conclusion of Lecture I : all those adaptations in the organism are purposeful, 

 which contribute to its maintenance, and insure its existence. This is not to say 

 that every individual irritability is at once or absolutely decisive for the life of an 

 organism, since organisms are fortunately not so perfectly adapted to external 

 conditions; but we may probably say that, on the whole, the possibility of life 

 depends on the co-operation of the irritabilities of the various organs. To 

 emphasise one point only ; how could an ordinary terrestrial plant live if its roots 



