174 BOTANY PAKT i 



of gravity is to be sought in the fact that gravity acts upon living 

 substances, not only physically but also in another way, as a stimulus 

 which induces a response in the internal activities of the plant body. 

 In these particular experiments it is the processes and forces of growth 

 which are locally increased or hindered by the action of gravity, and 

 produce results which do not correspond either qualitatively or 

 quantitatively with the known operations of the laws of gravity. 

 Living substance is dominated by the operation of stimuli. Irritability 

 is its most important attribute, for it is irritability alone that renders 

 possible what we call life. 



By irritability is meant the undoubted, though not fully under- 

 stood, causal connection between a particular influence and the 

 peculiar vital response of an organism. The disproportion that may 

 exist between an influence and its ultimate effect is plainly apparent in 

 a steam engine or in the firing of firearms. The slight pressure of the 

 finger in firing a cannon has as little correspondence, either quantita- 

 tively or qualitatively, with the destructive effect of the shot, as the 

 small effort necessary to open the throttle- valve of a locomotive has to 

 the continuous motion of a heavily laden goods train. The opening of 

 the valve of an engine before the steam is up has no effect ; it is only 

 when, by this process, the compressed steam is liberated that it is 

 followed by such enormous results. In the engine the connection 

 between the external influence and its effect is known ; in the effects 

 of stimuli on protoplasm this connection is not apparent, for in the 

 protoplasm the intermediate processes remain invisible to the eye, 

 even when aided by the best microscope. There is, however, no 

 occasion for the supposition that the connection between the 

 stimulating cause and its effect on the protoplasm is accomplished 

 by processes which are otherwise foreign to the protoplasm itself, and 

 which are called into existence only under the influence of a special 

 force, the vital force. It was formerly thought necessary to ascribe 

 not only all indications of life, but even all the transforming processes 

 carried on within animate objects, to the effects of a special vital force. 

 Now, however, the conception of the vital processes has become so 

 modified as no longer to require the supposition of such a special 

 vital force ; while the impossibility of explaining the manifold variety 

 of their manifestation by the action of a single force, and the advances 

 made in chemistry (cf. pp. 4, 5), have shown the futility of such a 

 supposition. 



Although, at the present time, the existence of a special, inde- 

 pendent vital force is denied by Physiology, and only such agencies 

 are accepted as are inherent in the substance of an organism itself, 

 still we must at the same time take account of such a vital force in 

 so far as it may be regarded as the expression of a living substance, 

 endowed with a peculiar, internal structure, which is in some way so 

 constituted that certain actions and conditions are followed by definite 



