ACTION OF LIGHT ON VEGETATION. 
755 
The retarding effect of lighf 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 
(Cucurbita, Ferdi'nanda, Nicotiana)^ 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 Hght 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 Hght, 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- 
cheriä), &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 heHotropic 
curvature ; z'. 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 Würzburg. 1872. 
2 Compare infra, Chap, IV. Sect. 20 ; also Arb. d, bot. Inst. Würzburg, Heft III. 
^ Further details on heliotropism will be given in Chap. IV. 
* 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. 742). 
3 C 2 
