378 



VEGETAELE lEEITABILITY. 



oscillatory movement the leaf becomes inclined in various ways, often 

 assuming a remarkably oblique direction. The upward and downward 

 movements seem to depend on the influence of light and darkness. 



Kg. 668. 



Pig. 669. 



During the day the leaf becomes more or less horizontal, while during 

 darkness it hangs down. Similar movements are seen in other species 

 of Desmodium, as D. gyroides and vespertilionis. 



The movements in these' cases have been referred to certain 

 changes in the organs, causing distension or contraction of the tissues. 

 Dutrochet and Morren refer them to alterations in the circulation of 

 fluids and air in the vessels and cells. In plants|with irritable leaves 

 there are freque^tly swellings where the leaflets join the stalk, as well 

 as where the stalk joins the stem. These swellings contain cells 

 which differ in their dimensions and their contents, and the move- 

 ments are considered as being produced by changes in the contents of 

 the cells, some of which become more distended than others, and thus 

 cause incurvation or folding. In these swellings the vascular bundles 

 are disposed in a circle near the periphery, and may be concerned in 

 the movements. The contraction of the protoplasm in the cells may 

 also be concerned in the leaf movements. Mechanical and chemical 



Fig, 663. Wood-Sorrel (Oxcdis Acetosella), with its temate leaves, wMch are said to dis- 

 play a certain amount of iiritaMlity when exposed to bright sunshine. During the night 

 eaeh of the thi-ee leaflets, forming the compound leaf, fold on their midrih, and then fall 

 down towards the common petiole. Some say that this plant is the true Irish Shamrock. 

 Fig. 659. A portion of the branch and leaf of the moving plant of India (Hedysarwii or 

 Desmodium gyrcms). The leaf is impari-pinnate, and often pinnately-trifoliolate. The large 

 odd leaflet, a, becomes more or less horizontal, under the influence of light and heat, and 

 is depressed during darkness or cold. Besides the movement of rising and falling, it has 

 also a lateral oscillatory motion, so that it often becomes oblique in its position relative to 

 the leaf-stalk. At its base there is a cellular intumescence. The smaller leaflets, 6, of 

 which there are either one or two pairs, have also swellings at their base. They exhibit 

 constant jerking movements, by which they approach and retire from each other, and these 

 motions go on to a certain extent during darkness. 



