ACTION OF LIGHT ON VEGETATION. 757 



endowed with motio7i^. The leaf-blades of Leguminosae, Oxalidese, Maranlaceae, 

 Marsiliaceae, &c. are borne on modified petioles which serve as contractile organs, 

 bending upwards or downwards under various external and internal influences, 

 and thus giving a variety of positions to the leaf-blades. If these plants are 

 placed in permanent darkness, the curvatures due to internal changes alternate 

 upwards and downwards. Light exercises an immediate influence on these peri- 

 odically contractile organs ; any increase of its intensity tends to give the blade an 

 expanded position, such as it occupies in the day-time ; any diminution tends to 

 cause it to assume a closed position upwards or downwards such as it has in the 

 night. This effect, which I formerly termed ' the paratonic action of light,' is not 

 the cause of the periodic movements; but rather counteracts the periodicity caused 

 by the internal forces. In most leaves endowed with periodic movements the para- 

 tonic influence of light is so strong that it neutralises them, and induces in their 

 place a periodicity dependent on the alternation of day and night. In the lateral 

 leaflets of the leaves of Desmodiuni gyrans on the contrary the internal causes of the 

 rapid periodic oscillations are so powerful as to overcome the paratonic action of 

 light; and these leaflets move upwards and downwards when the temperature is 

 high even in spite of changes in the amount of light. My earlier researches ^ show 

 that it is only the more refrangible rays that produce a paratonic effect, while red 

 rays act like darkness. 



The influence of light on the position of the contractile organs is not however 

 only of this direct character; the motile condition is also indirectly dependent on 

 it. Both the periodic and paratonic movement, as well as that {Mimosa) due to 

 mechanical irritation— in fact, the power of movement — is lost when the leaves 

 have remained in the dark for a considerable time, such as a whole day ; in other 

 words, they become rigid by long exposure to darkness. From this rigid condition 

 they do not immediately recover when again exposed to light ; the exposure to light 

 must continue for a considerable time, some hours or even days, before the motile 

 condition which I have termed ' Phototonus ' is restored. It is only in this condition 

 that the leaves are motile and sensitive to changes in the intensity of the light 

 or to mechanical irritation. The paratonic curvatures of fully developed contractile 

 organs caused by the action of light are distinguished from the heliotropic curvings 

 of growing organs by the fact that, firstly, they are connected with phototonus, 

 while the latter are not; and secondly, that they always take place in a plane 

 determined by the bilateral structure, while the plane of heliotropic curvature depends 

 only on the direction of the rays of light. 



'\The Chemistry of Chlorophyll. Gautier (Comptes Rendus, 1879, and Bot. Zeitg. 1880) 

 and Hoppe-Seyler (Ber. deut. chem. Ges. 1879, and Bot. Zeitg. 1879) have succeeded in 

 obtaining green crystals on the evaporation of an alcoholic solution of chlorophyll. 

 Gautier considers these crystals to consist of chlorophyll, but Hoppe-Seyler is of opinion 

 that they are a modification of chlorophyll, to which he gives the name of chlorophyllan. 

 The following are the analyses : — 



^ See Sachs, Ueber voriibergehende Starrezustande, &c., Flora, 1863.— Further details will be 

 given in Chap. IV. [Also Darwin, Movements of Plants, 1880.] 



^ Sachs, Ucber die Bewegungsorgane von Phaseolus und Oxalis, Bot. Zeit. 1857, p. Sii et tcq. 



