THE INFLUENCE OF LIGHT ON LIFE. 239 



The tendency of the growth of a plant is always in the direc- 

 tion of the most favourable light. Leaves and twigs all grow in 

 such a manner that they will not, in an undue degree, interfere 

 with the proper supply of light to the other leaves and twigs on the 

 same shrub or tree. We all know that when we look beneath the 

 surface of a clump of ivy or thick bushy shrub, we find that the 

 green leaves are mostly all outside, spreading themselves out in the 

 sunshine. Anyone who has grown a geranium in the house knows 

 very well that the leaves, and indeed the whole plant, bend 

 towards the window in search of light, and microscopists know that 

 if a tube containing Volvox globator be left standing in the room 

 for a short while, all the organisms wall congregate on that side of 

 the tube which is turned towards the light. In the same way does 

 vegetation everywhere grow according to the light, and take up a 

 position in the shade or in the sun according as its nature demands. 



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Fig. I. Fig. 2. Fig. 3. 



Where an excess of carbonic acid is present in the air, a plant 

 can bear a greater amount of sunshine than is possible under ordi- 

 nary circumstances. Many plants, however, are unable to stand 

 an excessive light and so perish, but some species are provided 

 with protection. For instance,* on the Mediterranean shores, the 

 leaves and stems of plants which grow on exposed situations are 

 provided with coverings of hair, whereby the green is almost 

 hidden from sight, which serve as a protection from the fierce 

 light of the sun. In some plants a violet colouring matter is found 

 near the surface of the leaves, so that, coming between the chloro- 

 phyll and the light, the excess of illumination is counteracted. 

 Many other devices with the same object have been observed, but 

 these will suffice for our purpose. That these hairs and the colour- 

 ing matter are for protection from excess of light is evident, for 

 when the same species are grown in diffused light, the hairs and 



*See Kerner and Oliver. 



