292 BOTANY PART I 



the result of the action of forces acting on growth. These move- 

 ments were therefore confined to organs, or parts of organs, still in a 

 state of growth. In contrast to the almost universal immobility of 

 all fully-grown organs, it is interesting to find that some plants have 

 found a means of carrying on vigorous movements without the assist- 

 ance of growth. 



It has already been shown (p. 179) that through the pressure of 

 increasing turgidity the elastic cell walls become greatly distended 

 and the cell cavity expanded, while, on the other hand, the cell 

 walls shrink and the cell becomes smaller when the turgor is 

 diminished (Fig. 174). It is to these changes in volume, which thus 

 result from alterations in turgor, that the varying movements of 

 fully developed living organs are due. 



Such variation movements occur only in foliar organs (foliage 

 and flower leaves, stamens, style and stigmatic lobes). These move- 

 ments are especially noticeable in the compound leaves of the 

 Leguminosae and Oxalideae, and also in the leaflets of Mtirsilui. In 

 the motile regions of these leaves special masses of tissue are, both 

 physiologically and anatomically, adapted for the promotion of this 

 form of movement. 



This tissue appears externally as a firm cushion or PULVINUS, sharply distin- 

 guished from the rest of the leaf-stalk, and is the direct cause of the leaf move- 

 ments. Anatomically considered, the pulvinus consists, for the most part, of 

 strongly turgescent parenchyma with very elastic cell walls. The vascular bundles 

 and mechanical elements, which, in other portions of the leaf-stalk, are situ- 

 ated towards the circumference, unite in the pulvinus in the form of a single flex- 

 ible strand, and so offer little opposition to the movement of the leaf resulting from 

 the curvature of the motile region (p. 182). The unfavourable arrangement of the 

 mechanical tissues is compensated by the turgescence of the pareuchymatous 

 tissues 6n which the support of the leaf-blade in its proper position in these cases 

 depends. The parenchyma of the pulvinus forms a thick enveloping layer about 

 this axial strand, by means of which, through the pressure arising from a differ- 

 ence in the turgescence of its opposite sides, a movement of the whole leaf-blade 

 is brought about, similar to that of the outspread hand by the motion of the wrist. 



i 



These variation movements are either autonomic, when the varia- 

 tions of turgor are due to no recognisable external influence, or 

 paratonic (aitionomic), when the turgor is regulated in a definite 

 way by the action of external stimuli. 



Autonomie Variation Movements. A remarkable example of 

 this form of movement is furnished by the small lateral leaflets of 

 Desmodium (Hedysarum) gyrans, a papilionaceous plant growing in the 

 damp Ganges plains and still more strikingly, according to 

 MOLISCH, by Oxalis hedysaroides. In a moist, warm atmosphere (22- 

 25) these leaflets make circling movements which are in no way 

 disturbed by variations in the intensity of the light, and are of such 

 rapidity that the tips describe a complete circle in 1-3 minutes; in 



