excitability and movement of the Leaves q/'Oxalis. 393 



chyma is formed of ovoid al cells, placed transversely, and of 

 such a development that two of them are equal in diameter 

 to a merenchymatous cell of the inferior cuticle which is 

 equal to three or four fifths of a tabular cell of the superior 

 cuticle. 



It follows from this structure that the cells of the infe- 

 rior mesophyllum are double the size of those of the upper 

 mesophyllum. The diachyma is moreover very rich in chlo- 

 rophyllum and in round clusters of crystals, occupying the 

 axis of the cells. 



It seems to me evident that analogy with the other plants 

 which are moveable by excitation, should lead us to place 

 the cause of the incurvation of the blade in the inferior meso- 

 phyllum, the cells of which by turgcscence elongate the in- 

 ferior pagina of the leaf, and thus cause the upper pagina or 

 the mesophyllum to fold upwards. The cellular tissue is here 

 also the essential organ of movement, and each cell a body 

 turgescent by excitability. 



The midrib is very large in this plant ; it is three or four 

 times larger than the secondary nerves, and it extends 

 straight and rigid from the basis of the leaflet to its apex. It 

 is transparent and juicy. This nerve reminded me of the 

 structure which I discovered in former dissections in the 

 Dioncea muscipula. 



Its cuticle is formed of little cells as high as they are wide, 

 nearly cubical, with very strong parietes. Four or five cor- 

 respond in width to the diameter of a single infrajacent cell. 

 Such a structure itself enables this cuticle to follow all the di- 

 latations that its interior mass can undergo. Directly within 

 this cuticle there occurs a cellular plane greatly developed, 

 formed of large cells, irregularly merenchymatous, with 

 strong parietes, and leaving between them passages, the 

 section of which is a triangle. There is little chromule, but 

 intracellular fluid in abundance. Each cell is the double 

 of those of a more interior cellular plane, and the quadruple or 

 the quintuple of those of the external cuticle. This plane of 

 great cells has them four or five in a row. Then come towards 

 the upper part of the midrib some chromuliferous cells, which 

 immediately surround a channeled plane of vessels, a channel, 



Ann. Nat. Hist. Vol. 4. No. 26. Feb. 1840. 2 f 



