IRRITABILITY. 533 



by Pfeffer, and a distinction between these and those of 

 growing organs has also been drawn by de Vries who, de- 

 signating the movements of variation by the equivalent 

 term " allassotonic," speaks of those of growing organs as 

 " auxotonic." 



The movements of variation are most commonly ex- 

 hibited by foliar organs. In the case of foliage-leaves the 

 movement is effected by means of a special motile organ, the 

 pulvinus, which is situated at the insertion of the leaf. This is 

 a swelling consisting of a mass of rather small- celled paren- 

 chymatous tissue covered by the epidermis and traversed by 

 a delicate fibrovascular bundle. The fibrovascular bundle 

 may be central so that there is an equal thickness of paren- 

 chymatous tissue above and below it, as in Oxalis Acetosella, 

 or excentric, as in Mimosa pudica, where there is more paren- 

 chymatous tissue below than above the bundle, the pro- 

 portion being as 6 : 7. In some cases, as in Oxalis Acetosella, 

 the walls of the parenchymatous cells of the pulvinus are 

 all of uniform thickness, whereas in Mimosa pudica the cells of 

 the upper half of the pulvinus have rather thicker walls than 

 those of the lower. There are also intercellular spaces in the 

 parenchymatous tissue of the pulvinus, which are largest in 

 the neighbourhood of the fibrovascular bundle, but diminish 

 in size in the outer layers of the tissue, until, in the most 

 external, they are altogether absent. The structure of the 

 whole motile portion of motile floral leaves, such as petals, 

 stamens, or styles, is essentially the same as that of the pul- 

 vinus, as is also that of the tentacles of Drosera, except that 

 in some cases, as in the staminal filaments of Berberis and 

 Mahonia, the intercellular spaces are absent (Morren, Unger), 

 and the tentacles of Drosera (see Fig. 33, p. 248). According 

 to Unger and Pfeffer, intercellular spaces are present in the 

 staminal filaments of the Cynareae. 



Of these movements of variation some are spontaneous. 

 The most marked instance of spontaneous movement is 

 afforded by the two lateral leaflets of the trifoliolate leaf of 

 Desmodium (Hedysaruiri) gyrans, known familiarly as the 

 Telegraph-plant, first observed by Lady Morrison, and first 



