6jH GENERAL CONDITIONS CF PLANT-LIFE. 



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 Desmodium gyrmis on the contrary the internal causes of the 

 rapid periodic oscillations are so powerful as to overcome the paratonic sensi- 

 tiveness; 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 excite paratonic sensitiveness, 

 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 th^.t of INIimosa when 

 mechanically irritated — in fact, the power of movement in plants — is lost when 

 they 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 ex- 

 posure to light must continue for a considerable time, some hours or eveii 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 sensitiveness to 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 ahvays 

 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. 



Optical Properties of the Colouring matter of Chlorophyll. If the parts of plants that 

 contain chlorophyll are repeatedly boiled in water and then quickly dried at a temper- 

 ature not too high and pulverised, a substance is obtained which is easily examined 

 and can be preserved for a long time unchanged. From this powder the green colouring 

 matter can be extracted by alcohol, ether, or oil. The green solution is speedily 

 changed by the action of light in proportion to its intensity, the less refrangible rays of 

 the spectrum acting most actively and rapidly. It then assumes a dirty brownish yellow- 

 green colour, the green colouring matter having become modified or lost its colour. 



If sunlight that has passed through a stratum of the pure green solution not 

 too thick or too dark is decomposed by a prism, an extremely characteristic spectrum 

 is obtained in which rays of very various refrangibility appear to have been more strongly 

 absorbed the darker the solution or the thicker the stratum. This chlorophyll-spectrum 

 has been the subject of much research ; the most recent and comprehensive being that 

 of Kraus, from whose description I borrow the following^: — 



The spectrum of an unchanged alcoholic solution of chlorophyll shows seven 

 absorption-bands, four of which are narrow (Fig. 447 A, I, II, III, IF), and are situated 

 in the less refrangible half; while three {F, FI, FII) are broad and are situated in 

 the more refrangible half. The latter, distinguishable as distinct bands only in very 

 dilute solutions, coalesce, even in the solutions of medium concentration which are 



^ Sachs, Ueber die Bewegungsorgane von Phaseohis und Oxalis, Bot. Zeit. 1857, P- Sii et seq. 



^ Kraus. Sitzungsb.der yjhys.-med. Soc. in Eilangen, June 7 and July 10, 1871. See also Askenasy, 

 Bot. Zeit. 1867, P- 225; Gerland und Rauwenhoff, Archives neerlandaises, vol. VI 1H71 ; and 

 Gerland, Pogg. Ann. 1871, p. 585. [Kraus, Zur Kenntniss der Chloroph) llfarbstoffe u. ihrer 

 Vcrwandten; Stuttgart, 1872. F"or reference to Mr. Sorby's papers see Sect, b a.] 



