ACTION OF LIGHT ON VEGETATION. 755 



The retarding effect of light on the growth of the shoot is evident even in 

 a short time ; and, as I have already briefly shown ^ a periodical oscillation in the 

 rapidity of growth is caused by the alternation of day and night (when the tem- 

 perature is nearly constant). This variation is shown by the growing internode 

 exhibiting a maximum of hourly growth towards sunrise, decreasing gradually 

 from the advent of daylight till mid-day or afternoon, when it reaches its 

 minimum, and increasing from this time till morning, when it again attains its 

 maximum. 



Prantl ^ has shown that a similar periodicity exists in the growth of leaves when 

 day and night alternate normally. The fact that the leaves of the same plants 

 {Cucurh'/a, Ferdinafida, Nicoliana), when they become etiolated by remaining in 

 continuous darkness, are much smaller, is in apparent contradiction to this. But 

 in such a case we have not to do with leaves which are healthy and which are 

 periodically assimilating, but with sickly leaves which contain no chlorophyll. These 

 small yellow leaves, developed in darkness, are not exposed to the favourable 

 influence of light upon which assimilation and its effect upon growth depend in 

 normal leaves. 



One of the best-known phenomena occasioned in plants by light is the fact that 

 growing stems and leaf-stalks, when the amount of light which they receive is very 

 different on different sides, bend or become concave towards the side exposed to 

 the most intense light. This curvature is caused by the slower growth in length 

 of the illuminated than of the shaded side ; and parts of plants which show this 

 behaviour to light are called heliotropic^. From the fact of heliotropic curvature 

 towards the side which receives the most light, it is obvious that the plant would 

 grow more quickly if shaded on all sides than if the light were more intense. 

 The observation that leaves, some roots. Fungi, filamentous Algae (like Vau- 

 cheria), &c., curve heliotropically, indicates that their growth is retarded by light. 

 That the chlorophyll has no share in causing this heliotropism is shown by the 

 fact that organs which contain none, like some roots, or Fungi, as the perithecia 

 of Sordaria fimiseda (according to Woronin), the stipes of the pileus of Clavi- 

 ceps (according to Duchartre*), and colourless etiolated stems, bend towards a 

 stronger light. Since most heliotropic parts of plants are highly transparent, the 

 light which falls on one side must penetrate more or less to the other side, on 

 which also some light falls ; it follows therefore that even inconsiderable differences 

 in the intensity of the Hght which falls on the two sides must cause heliotropic 

 curvature; i.e. difference in the rate of growth ^ If plants which show heliotropic 

 properties are grown in a box which receives light from one side that has passed 

 in one case through a solution of potassium bichromate, in another case through 



^ Sachs i Heft II of the Arbeiten des Bot. Inst, in Wiirzburg. 1872. 



2 Compare infra. Chap. IV. Sect. 20 ; also Arb. d. bot. Inst. Wiirzburg, Heft III. 



^ Further details on heliotropism will be given in Chap. IV. 



* Diichartre, Compt. rend. 1870; vol. LXX. p. 779. 



5 It must however be noted that in the case of parts containing chlorophyll the light in pene- 

 trating the tissues loses its more refrangible rays which are the only ones that produce the effect ; as 

 lias been already shown, only the less refrangible rays pass through the superficial layers (see 

 p. 742). 



3 C 2 



