394 Prof. De Briffnoli and Prof. Morren on the 



-o 



the hollow of which is directed upwards, and which is filled 

 with little cells and sap vessels. 



This structure reminds us of that of the petiole of the Mi- 

 mosa pudica. The distention of the great cells of the lower 

 plane of the midrib must force the two half blades of the leaf 

 to approach each other ; and this enlargement, produced by 

 excitability and allowed by the intercellular passages, thus 

 becomes the proximate cause of the folding up of the two lobes 

 of the leaflet of Ox alts Deppei all along the nervure. There is 

 the same mechanism and a very analogous structure in the 

 DioncEa muscipula. 



There is no pulvinus at the base of the leaflets of the Oxalis 

 as in the Mimosae, but there is a peculiar organization in this 

 part which answers the purpose of this organ. If we observe 

 attentively how the leaflet is articulated to the petiole on the 

 under side, we find that the midrib terminates in a crescent, 

 the concave of which faces the petiole. The petiole in its turn 

 ends in another crescent, the concave of which faces the leaflet; 

 so that the partial petiole, which is so short as not to exceed 

 a millimetre and a half, is terminated by two opposite cre- 

 scent-shaped articulations, the convexities of which face each 

 other. Thus much for the under part of the leaf. 



As for the upper part, the two margins of the leaflet which 

 converge at the base of the leaflet to form the point of the 

 heart, become imperceptibly thicker and unite to form a kind 

 of crescent-shaped bridle, whose concave is turned towards 

 the leaflet. The common petiole receives in its turn the par- 

 tial petiole by a crescent-shaped articulation, but which, in 

 this instance, has its concave turned towards the leaflet, that 

 is to say, it is a crescent parallel to the first. Between them 

 spreads a red cuticle, which is strongly plaited crosswise. 



The transverse section of this organ gives that of a depressed 

 cylinder formed of a strongly resisting cuticle, consisting of 

 ovoidal cells lying flatwise, the parietes of which ai'e of the 

 thickest. Then comes a fully developed layer of cellular tis- 

 sue with cells plainly merenchymatous, forming at least a 

 dozen rows. Each cell has a central mass of chromule. There 

 are fewer rows of cells (from 8 to 9) towards the upper part 

 of the partial petiole. In the centre of this, but a little higher 



