THE VITAL PROPERTIES OF THE CELL 93 



or pressure. Similarly, as Sachs has pointed out, plant cells also 

 are furnished with their specific energies. Tendrils and roots 

 bend themselves in a manner peculiar to themselves, whether 

 stimulated by light, gravitation, pressure, or electricity. The effect 

 of a stimulus bears the specific stamp, so to speak, of the special 

 structure of the stimulated substance, or, in other words, irritability is 

 a fundamental property of living protoplasm., but it manifests itself in 

 specific actions according to the. specific structure of the protoplasm 

 under the influence of the external world. 



The same idea is expressed by Claude Bernard (IV. la) in the 

 following words : " La sensibilite, consideree comme propriete du 

 systeme nei-veux, n'a rien d'essentiel ou de specifiquement distinct; 

 c'est I'irritabilite speciale au nerf, comme la propriete de contrac- 

 tion est I'irritabilite speciale au muscle, comme la propriete de 

 secretion est I'irritabilite speciale a 1'element glandulaire. Ainsi, 

 ces proprietes sur lesquelles on fondait la distinction des plantes 

 et animaux ne touchent pas a leur vie meme, mais seulement aux 

 mecanismes par lesquels cette vie s'exerce. Au fond tous ces me- 

 canismes sont soumis a une condition generale et commune, 

 I'irritabilite." 



In speaking generally of irritability, another peculiar pheno- 

 menon deserves especial attention, namely the transmission or con- 

 duction of stimuli. If a small portion of the surface of a 

 protoplasmic body is stimulated, the effect produced is not limited 

 to this point alone, but extends to far outlying ones. Hence the 

 changes produced by the stimulus at the point of contact must be 

 more or less quickly shared by the rest of the body. Stimuli, as 

 a rule, are more quickly transmitted in animal than in vegetable 

 bodies ; in human nerves, for example, the rate is 34 metres per 

 second ; it is always slower in plant protoplasm. 



We imagine that the substance which is capable of receiving 

 stimuli forms a system of exceedingly elastic particles in a condi- 

 tion of unstable equilibrium. In such a system it is sufficient for 

 one of the particles to receive a slight shock, in order to set all the 

 others in motion, since each transmits its movement to another. 

 This theory explains the phenomenon, that exceedingly great effects 

 are often produced by very slight stimuli, just as a small spark, 

 by setting on fire a single grain of powder, may cause a powder 

 magazine to explode. 



Finally, another peculiarity of organic matter is its capacity of 

 returning more or less completely to its original condition, after a 



