SPONTANEOUS MOVEMENTS IN PLANTS. 347 



676. In all these cases, whether of slow or rapid change of posi- 

 tion, the immediate cause of the movement, however incited, must 

 be either the shortening of the cells on the concave side, or their 

 elongation on the convex side. The fact that stems curved towards 

 the light tend to curve still more when the convex side is cut 

 away (669) points to a contraction of the cells on the concave side 

 as the cause of the curvature. The elastically bursting pods of the 

 Balf^iUTi or Touch-me-not (Impatiens), &c. confirm this view. Here 

 the valves of the capsule curve inwards very strongly when liber- 

 ated in dehiscence ; and that this is owing to the shortening of the 

 cells of the inner layer, and not to the enlargement or turgescence of 

 those of the thick outer layer, is readily shown by gently paring 

 away the whole outer portion before dehiscence ; for the inner layer 

 when liberated still incurves and rolls itself up as strongly as before. 

 The short valves at the summit of the pod of Echinocystis slowly 

 curve outwards in dehiscence ; here the cells of the outer layer of 

 the valve are longer and narrower than those of the inner, and 

 the latter are stretched and torn in opening ; so that here the con- 

 traction of the cells on the side which becomes concave is undoubt- 

 edly the cause of the movement. And since muscular movements 

 are effected by the contraction of the cells which, placed end to end, 

 compose a muscular fibril, we may suspect that vital movements 

 generally, both in vegetables and in animals, are so far analogous, 

 that they are brought about in the same general way, viz. by the 

 shortening of cells. Even the opening and closing of the stomata 

 of the leaves (268) appear to be controlled by the vital force, and 

 to be effected by a self-caused change in the form of the guardian 

 cells. How the light, or external irritation, or any other influence, 

 acts in inciting tliis change of form of the cells of some part of a 

 plant, we know no more, and no less, than we know how a nerve, or 

 an electrical current, acts upon a muscle of an animal to bring 

 about the contraction or change of shape of its component cells. 

 If animals make 



677. Spontaneous or Automatic MoTcments, so also do some plants 



execute brisk and repeated movements irrespective of extraneous 

 force, or even of extraneous excitation, and which, indeed, are ar- 

 rested by the touch. An instance of such spontaneous and contin- 

 ued motion, of the most remarkable kind, is furnished by the trifoli- 

 olate leaves of Desmodium gyrans, an East-Indian Leguminous plant. 

 The terminal leaflet does not move, except to change from the 



