52 ZOOLOGICAL PHILOSOPHY 



It is known that irritability is a faculty essential to the parts or to 

 certain parts of animals, and that it is never suspended or annihilated 

 so long as the animal is ahve and the part possessing it has suffered 

 no injiury. Its effect is seen in a contraction which takes place 

 instantly throughout the irritable part on contact with a foreign 

 body ; a contraction which ends with its cause, and which is renewed 

 whenever the part after relaxation is irritated by new contacts. Now 

 nothing of this kind has ever been observed in any other part of 

 plants. 



When I touch the extended branches of the sensitive plant (Mimosa 

 pudica), instead of a contraction I observe in the joints of the disturbed 

 branches and petioles a relaxation, which permits these branches and 

 petioles of the leaves to droop, and causes the leaflets themselves to 

 sink down upon one another. When once that sinking has been 

 produced it is useless to touch again the branches and leaves of this 

 plant ; no effect follows. A longish time is required, unless it is 

 very hot, for the distension of the joints of the small branches and 

 leaves of the sensitive plant ; when all these parts will again be raised 

 and spread out, ready to fall together once more upon a contact or 

 slight shaking. 



I cannot see in this phenomenon any relation to the irritabihty 

 of animals. I reflected however that during growth, especially 

 when it is hot, there are produced in plants many elastic fluids, 

 part of which are incessantly being exhaled. Hence I conceived 

 that in leguminous plants these elastic fluids might accumulate, 

 especially in the joints of the leaves, before being dispelled, and that 

 they might then distend these joints and keep the leaves or leaflets 

 spread out. 



In this case, the slow dissipation of the elastic fluids in question set 

 up in leguminous plants by the approach of night ; or the sudden 

 dissipation of the same fluid set up in Mimosa pudica by a shght shaking, 

 will give rise for leguminous plants in general to the phenomenon 

 known under the name of sleep, and for the sensitive plant to that 

 wrongly attributed to irritabihty.^ 



It follows from the observations which I shall set forth below, and 

 from the inferences which I have drawn from them, that in general 



• I have developed in another work {Hist. Nat. dm vegetaux, edition D^tendlle, 

 vol. i. p. 202) other analogous phenomena observed in plants such as Hedysarum 

 girans, Dionaea muscipula, the stamens of the flowers of Berberis, etc. ; and I have 

 shown that the curious movements observed in the parts of certain plants chiefly 

 in hot weather are never the result of a real irritability essential to any of their fibres ; 

 but that they are sometimes hygrometrio or pyrometric effects, sometimes the results 

 of elastic relaxations which take place under certain circumstances, and sometimes 

 of a swelling and drooping of parts by the local accumulations and more or less rapid 

 dissipations of elastic and invisible fluids which are being exhaled. 



