148 VITALITY IN OXALIDS, [BOOK n. 



petiole which forms the axis. This last movement is similar 

 to that which takes place in the evening at the time of the 

 sleep of the plant, and which has caused these leaves to be 

 called dependent." 



M. Morren offers the following explanation of the pheno- 

 menon : 



" As in all plants moveable from excitation, the organs of 

 motion reside in the apparatus itself which moves. Now 

 here the apparatus consists of: 1. The blade itself of the leaf, 

 an organ of incurvation. 2. The large midrib. 3. The 

 partial petiole ; the former being an organ for folding back, 

 the latter an organ of incurvation. Now, the blade of the 

 leaf is composed, above, of a cuticle with pinenchymatous cells, 

 that is to say, tabular-shaped (Meyen) ; beneath, of a cuticle 

 with merenchymatous cells, swollen up, like bladders, with 

 numerous small linear stomata between all the raised cells, 

 so that one amongst them is often surrounded by six stomata; 

 in the middle by a double diachyma, whose upper plane is 

 formed of prismatic or ovoid al cells placed perpendicularly, 

 and of such a size that upon the length of a single tabuliform 

 cell of the upper cuticle (derme) there are six utricules of the 

 diachyma. The plane of the diachyma is formed of ovoidal 

 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 inferior mesophyllum are double 

 the size of those of the upper mesophyllum. The diachyma 

 is, moreover, very rich in chlorophyllum 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 move- 

 able by excitation, should lead us to place the cause of the 

 incurvation of the blade in the inferior mesophyllum, the cells 

 of which by turgescence elongate the inferior 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 exci- 

 tability " (Annals of Natural History, vol. iv.) ; or, as we 

 should say, by vitality. 



