652 PHYSIOLOGY. 



construction of different layers also promotes the process, as well as the 

 pressure from within of the growing seeds. 



The movements of the awn of Avena sterilis are in like manner attri- 

 butable to the possession of two layers of different anatomical and hygro- 

 scopical structure. The similar twisting of the awns of other Grasses, 

 the styles of Erodium, &c., has been described by Mr. F. Darwin, who, in 

 the ' Linnean Transactions,' has explained the mechanism by which these 

 plants are enabled to bury themselves in the ground. 



Irritability. The equilibrium may be disturbed by various stimuli, 

 mechanical and chemical. When an organ is strikingly affected by 

 mechanical influences, we have the phenomenon of u irritability/' such 

 as we see in the Sensitive Plants, Dioncea, &c. The "sleep" of plants 

 is doubtless a phenomenon differing only in degree ; and this slower move- 

 ment is probably attributable to the chemical action of light. 



Periodic and Induced Movements. The movements of plants consist 

 for the most part of the curvature or the folding-up of organs ; and in 

 such cases the organs are always found to possess certain peculiarities 

 of structure and mode of union to other organs. The movements take 

 place periodically (in consequence of the regular alternations of external 

 influences), or irregularly (from the accidental influence of special stimuli). 



Periodical movements are more particularly connected with the influ- 

 ence of heat and light ; and their degree is generally more or less propor- 

 tionate to the intensity of these influences. 



Action of Heat and Light. The sensitiveness and periodic movements 

 of the leaves of the Sensitive Plant are not manifested at a lower tem- 

 perature" than 15 C., while permanent loss of motion and death occur 

 above 52 C. 



Exposure to light causes movement in the leaves of Sensitive Plants, 

 raising them from the depressed position they occupied in darkness ; and 

 prolonged obscurity induces rigidity and loss of motion. Thus Bert 

 showed that Sensitive Plants kept in the dark lost their sensibility after 

 seven days, and died in twelve days. Under green light the plants died 

 after sixteen days exposure, retaining their sensibility for twelve. Under 

 violet and blue light the plants existed (but did not grow) for three 

 months, and retained their sensibih'ty. In Pothos scandens with dilated 

 leaf-stalks jointed to the blade, movements may be seen which are appa- 

 rently associated with the influence of light ; thus on the growing shoot 

 the position of the leaves and leaf-stalks varies according to the direc- 

 tion of the light. In some cases lamina and petiole are in the same plane, 

 at other times at right angles one to the other ; and even the petiole is 

 twisted on its own axis, so as to place the two surfaces of the leaf in a 

 more or less vertical position. Every intermediate position may be 

 observed. 



Effect of Turgescence. In the best known Sensitive Plant, Mimosa 

 pudica, there is a swelling at the ba,ae of the petiole, the cells of which con- 

 stitute, as it were, two springs acting in contrary directions, so that if the 

 one, from any catise, be paralyzed, the other pushes the leaf in the direction 

 of least resistance. These springs, if they may be so called, are set in action 

 by the rush of fluid creating a turgid state of the one set of cells, and a 

 relatively empty state of the other series. What circumstances regulate the 



