6/6 GENERAL CONDITIONS OF PLANT-LIFE. 



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. 



Since the leaves of etiolated plants are much smaller than in the normal state, 

 it might be expected that they would grow much more quickly in the day than in the 

 night, or that the mechanical laws of their growth would be opposed to those of 

 the internodes with respect to the influence of light. But it would be too hasty 

 to come to this conclusion; for the objection might be made that normal leaves 

 assimilate in the day, while they grow chiefly in the night \ 



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 

 diff'erent 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 helioiropic'^. 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 peri- 

 thecia oi 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 light 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 

 one of ammoniacal copper oxide, the internodes of the first remain quite straight 

 and lengthen considerably as if they were in the dark, while those exposed to the 

 mixed blue light grow less and at the same time bend strongly towards the light. 

 It follows from this that only rays of high refrangibility, the blue, violet, and 

 ultra-violet, cause the curvature by retarding growth^. 



^ Compare infra, Chap. IV. Sect. 20. 



2 Further details on heliotropism will be given in Chap. IV. [See also p. 190.] 



3 Duchartre, Compt. rend. 1870; vol. LXX, p 779. 



* 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 

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

 p. 665). 



^ See Sachs, Bot. Zeitg. 1865, On the action of coloured light on plants, where the literature is 

 also quoted. I consider experiments with absorbent fluids more decisive than those with the spec- 

 trum ; in this latter Guillemin states that not only do all the rays act heliotropically, but that there 

 is even a lateral curvature towards the blue end of the spectrum. When the light is sufficiently 

 strong the spectrum is certainly never free from diffused white light, which will cause heliotropism 

 even when its intensity is very small. 



