excitability and movement of the Leaves of Oxalis. 395 



than the geometric centre, are the air vessels (tracheae) below, 

 and the sap vessels above, surrounded by smaller and more 

 fully coloured cells. 



This organization is fundamentally that of the pulvinus of 

 Mimosa pudica. When the merenchymatous cells of the cor- 

 tical part of the lower zone are distended or turgescent, the 

 leaflets are horizontal ; when their turgescence stops and that 

 of the cells of the upper zone predominates, the leaflets droop, 

 as in the natural sleep of this Oxalis, and as takes place after 

 it has been subjected to disturbance. 



At any rate, the excitability of the cellular planes and of each 

 cell in particular, and the distention which is the manifestation 

 of it, must be admitted to account for the different positions 

 which the leaves of the Oxalis take when they are struck. 



The movement of the leaves of the Oxalis, although slower 

 than that of the sensitive plants, is also not on that account less 

 remarkable ; it is even so much the more interesting to us, as, 

 taking place in our indigenous plants, we can the better ob- 

 serve it ; the physiological study of our national species hence 

 obtains a new attraction, and the discovery of M. De Brignoli 

 and his pupils has led in its turn to the discovery of an ana- 

 logy of structure between the leaves of the Oxalideae and those 

 of the Mimosas ; an analogy which could hardly have been 

 expected, but which is fully proved by direct observation. 



The moveableness of Oxalis is the more singular, as M. De 

 Candolle has not been able to modify the sleep of these plants, 

 either by means of darkness or light, whence he concluded that 

 the movements of sleep and awakening were connected with 

 a periodical disposition of motion inherent in the plant*. We 

 see, however, that a simple blow makes the leaflets when awake 

 take the posture of sleeping leaflets. 



M. Virey, in his ( Considerations nouvelles sur Pacidite dans 

 les plantes irritablest/ has made evident by the recapitulation 

 of the species in which the movement of any organ has been 

 observed, that most of them were acid ; this is indeed a cu- 

 rious analogy to demonstrate, but which proves nothing, for 

 we cannot see what connexion there should be between a thing 



* Physiologie, vol. ii. p. 861. 



f Journal de Pharmacie, 1839, May, p. 289. 



2 F 2 



