793 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 



(6) Transitory rigidity caused by electrical agency'^ was observed by Kabsch in the 

 gynostemium of Stylidium. A weak current produced the same result as concussion ; 

 a stronger current destroyed the irritability, which however returned after half an hour. 

 In Desmodium gyrans, on the other hand, the leaflets which had been rendered rigid by 

 cold (22° C.) were again made motile by the action of an induction-current. 



Sect. 29. Mechanism of the Movements'^. We have now to explain in 

 what manner the movements we have been describing are effected in any particular 

 case in the normal and healthy condition of the organs. But we must first investigate 

 the anatomical and mechanical contrivances which have the power, under th'e in- 

 fluence of certain forces, of causing those changes in the tissue that result in the 

 movements in question ; and we must then enquire whence the forces are derived 

 which actually set in motion the contractile organs. That the forces which act 

 when the organs are in a state of tension are set in motion by a small impulse, is 

 evident from the fact that the movements are brought about by causes which could 

 only produce the effect by special contrivances, the motive power of the movement 

 being altogether out of proportion to the effect produced. The strong downward 

 curving not only of the large contractile organ of a Mimosa-leaf by a slight touch 

 on the under side, but the concurrent movement of other leaves, reminds one of 

 the behaviour of a steam-engine, the powerful forces of which are set in action by 

 a slight pressure on a valve. But the extraordinary transmutation of tension into 

 active force effected by light in the periodically motile parts of plants, causing them 

 to pass from the nocturnal to the diurnal position, cannot be made the subject of 

 so exact a comparison. This phenomenon may rather be compared to the power of 

 the rays of the sun to set fire to gunpowder placed in the focus of a burning-glass, 

 which on its part sets a machine in motion by the expansion of its gases, or propels 

 a cannon-ball in the barrel. 



As far as observations have at present been made, two kinds of forces exist in 

 the contractile organs of plants, which, by their mutual tension, cause both the 

 irritable and the periodically motile conditions of plants : on the one hand the at- 

 traction of water by the substances contained in the parenchymatous cells of the 

 tissue capable of expansion ; on the other hand, the elasticity of the cell-walls. 

 By the former the turgid parenchyma is strongly stretched until the elasticity of 

 the cell-wall reaches an equilibrium with the endosmotic force. From the arrange- 

 ment of the tense tissues (see Sect. 27) this state of equilibrium is necessarily 

 transitory ; every increase of the turgidity on one side must cause a curvature on the 

 other side, every diminution on one side a curvature tow^ards that side. Since it is 

 impossible to suppose that the elasticity of the cell-walls undergoes periodical 

 change, or is altered by variations in the amount of light or by slight concussion, 

 the only alternative left is to ascribe the alterations in the tension of the con- 



1870, is a record of a series of experiments on the effect of chloroform on the irritability of the sta- 

 mens of Mahonia. — Ed.] 



^ Kabsch, Bot. Zeit., 186 1, p, 358. 



^ The only general descriptions of these movements is Hofmeister's in Flora, 1862, No. 32 

 et seq., from which mine differs in important points; but I cannot enter here into a discussion of 

 the particulars of the difference in our interpretation of the phenomena. 



