vi ELECTROMOTIVE ACTION IN VEGETABLE CELLS 11 



till the cone-like protuberance of the parenchyma of the lobe is 

 reached, when the contact at once causes the leaf to fold together. 



Along with these mechanical irritants we must include the 

 abstraction of water, as a stimulus to the excitable parenchyma 

 at the base of the hair. Darwin (9) found, on placing the leaves 

 of Dioncea in concentrated solution of sugar, or even on applying 

 a single drop to an excitable hair, that the laminae closed up 

 immediately. Munk observed the same effect with alcohol 

 and concentrated, salt solution, and again when the plants were 

 exposed to rapid evaporation in dry air. 



The movements of the leaf of Dioncea, like all movements of 

 plants, are due to processes that cannot be compared with the 

 contraction phenomena of muscle, or with any true contractile 

 tissue. They are much rather special instances of cell co-ordina- 

 tion, depending on quite different mechanical principles. The most 

 characteristic type of plant-movement is the well-known Mimosa 

 pudica. Each leaf -stalk of the "sensitive plant" bears four 

 secondary petioles, provided in their turn with two rows of 

 leaflets. During the day the principal stalk is erect, the second- 

 ary petioles being spread out like the fingers of a hand. The 

 single leaflets are expanded into a smooth surface. At night the 

 leaves sink down, the secondary petioles are folded together, and 

 the leaflets stand up, so that the inverted surfaces are in contact. 

 This change of position in all the leaves can be induced in the 

 day-time also by violently shaking the plant, or bringing it into 

 an atmosphere of chloroform or ether vapour. Locally, however, 

 or within a limited tract, the same effects occur on mechanically 

 exciting the single leaflets (by touching, pricking, cutting), par- 

 ticularly at the point where the principal leaf-stalk is attached 

 to the stem. Here there is a cushion (pulvinus), which is simi- 

 larly developed at the base of the second and third petioles. In 

 every case an excitatory movement is discharged only on touching 

 the under side of the swelling at the joint, while the upper surface 

 is almost entirely non-excitable. 



Anatomical investigation of one of these swellings shows it to 

 be traversed by a vascular bundle with a layer of very succulent 

 cells between the bundle and the green cortex ; these cells are 

 tolerably thick-walled upon the upper (insensitive) side of the 

 joint, while on the lower side the walls are comparatively slender. 

 On bisecting the swelling transversely, a funnel appears on 



