LEAF-MOVEMENTS 



the leaf assumes the day-position. In the case of the Sensitive 

 Plant (Mimosa pudica, Fig. 56, B) a similar but very sudden 

 fall in turgidity results when the leaf is touched or injured. 



The leaves so far considered exhibit a marked difference of 

 structure and appearance between the upper and lower surfaces 

 (dorsiventral type), but this is not always the case. In the Iris 



FIG. 56. A, Pulvinus of Runner Bean (Phaseolus) and diagrams of trans- 

 verse sections of the pulvinus (Pu.) and the petiole (Pe.}. B, Pulvinus 

 of Sensitive Plant (Mimosa pudica). p., pulvinus ; s., stipules. 



and other Monocotyledons, where the leaf -blades stand vertically, 

 the structure is identical on the two sides (isobilateral type) . In 

 still other cases, particularly in succulents among British plants 

 (e.g. White Stonecrop, Sedum album), the leaves are more or less 

 cylindrical, and, apart from the dorsiventral arrangement of the 

 vascular bundles, exhibit radial organisation (Fig. 86, p. 165), 

 with a palisade layer extending uniformly round the periphery. 



