CHLOROPHYLL AND LIGHT INTENSITY. 395 



surfaces are never to be met with in shady places. On the floor of a thick wood 

 grow no irises and no compass-plants; these are at home on the ridges of rocky 

 mountains, and on treeless prairies, and if it happens that a seed of such a plant 

 falls intci the shade of a wood and germinates there, developing foliage-leaves, then 

 the leaf- surfaces do not assume a vertical position, and twist and bend themselves 

 until their broad surface is turned towards the scantily-penetrating difiuse light. 

 If the light falls from above through the interstices of the leafy covering, the 

 leaf-surface becomes horizontal and parallel to the ground; if the crests of the trees 

 close together to form a thick, opaque canopy, and the diffuse light penetrates from 

 the side between the trunks of the trees, the leaf-laminse bend and turn to the 

 openings of the wood, giving the impression that they are looking out longingly 

 to the sunny country which borders the dense, deep-shaded forest. 



The same thing is seen under every small shady bush, and, generally speaking, in 

 dll places where dissimilar tall plants overlap one another, and where the leaves of 

 the lower are arched over by those of the higher plants. If they belong to different 

 species, they cannot be said to have any consideration for one another. Each looks 

 out only for itself, and the lofty species do not trouble themselves about the 

 inferior stuff which arises from the soil under their leaves. If in the depths below 

 there are plants which find all they require in the diffuse light and the green rays 

 passing through the leafy roof, very well; if not, these lower plants must perish in 

 the shade. But it is otherwise if the leaves overlapping each other belong to one 

 and the same branch, to one and the same plant; where they must co-operate for 

 the weal of the whole plant, and the whole can only maintain itself in the struggle 

 for existence by harmonious division of labour. Therefore care must be taken that 

 no leaf shall take too much light away from another; that one shall protect and 

 support the other; that neighbours shall not jostle if one or the other has to bend, 

 turn, and extend itself in order to best adapt itself to the incident light. 



And this foresight actually occurs. It is exhibited, first of all, in the position 

 of the leaves on the stem, or in other words, in the regulation of the intervals 

 between the places of origin of neighbouring leaves; secondly, by the fact that the 

 stalks of the green leaf -blades have the capacity of rising and sinking, twisting and 

 bending, and also of elongating if required; and thirdly, through the form which the 

 leaf-surfaces possess. 



