THE NATURE OF IRRITABILITY u 



thereby definitely imply that we recognize them as the result of the 

 specific character of the organism, and as due to the particular combination 

 of natural forces which it represents x . 



We are necessarily led to this conclusion by the arguments that 

 induced us to reject the assumption of the existence of a special vital 

 force. Our knowledge of the essential nature of the living mechanism 

 is extremely meagre, and it is hence not surprising that we know very 

 little about the manner in which its manifestations of irritability originate. 

 For this reason it is often doubtful to what degree, and in what respect, 

 a particular vital activity is a manifestation of irritability. 



Such phenomena as the bending of a branch by the weight it supports, 

 or the distension of a cell by turgor, are directly due to mechanical forces, 

 but they may nevertheless be indirectly brought about by the operation 

 of an appropriate stimulus, influencing either the turgidity of the cell, 

 or the weight of the branch. The stimulus is a push as it were, to 

 which the organism responds according to its inherent nature and the 

 means at its disposal. Just as a ray of light may induce a man to use 

 his locomotory apparatus and go from darkness to the light of day, so 

 also may light influence the movements or direction of growth of plants ; 

 for in plants rooted to the soil the part above ground may bend and 

 grow towards the source of illumination, while in those which are free 

 swimming the entire organism may respond by a movement of translation 

 in the same direction. 



Stimuli may be external or internal in origin. In the latter cate 

 they are said to be autonomous, and arise from some obscure change in the 

 vital activities of the plant. External stimuli, on the other hand, always 

 act inductively, producing internal changes which may or may not be visibly 

 manifested. They are hence often referred to as inductive stimuli 2 . 



When the stimulus is internal in origin, it is usually less apparent I 

 than when it is external ; in the latter case its intensity may be varied / 

 at the will of the observer, and corresponding changes may be noticed 

 in the result produced. 



An internal stimulus may, in some cases, be as definite and clearly 

 determinable as an external one. This is the case when a particular 

 reaction can be traced to the production of an enzyme by the organism, 



1 From this point of view an exact and scientific comprehension of irritability is possible. 

 Since this is simply a special manifestation of energy, all attempts to prove that the existence of 

 irritability involves properties and forces totally foreign to the inorganic world must necessarily fail. 

 A short account of the progress of our knowledge and ideas in this direction will be found given in 

 Die Reizbarkeit der Pflanzen, 1893 (Verhandlung der Gesellschaft d. Naturf. und Aerzte). In the 

 first edition of this book, as well as in Osmot. Untersuch., 1877, p. 202, the same general principles, 

 as regards the nature of irritable processes, have been previously brought to notice. 



a Sometimes called ' trophic stimuli.' 



