394. CHLOROPHYLL AND LIGHT INTENSITY. 



From these culture experiments two things may be learned : first, that a very 

 brilliant light is able to influence the distribution of plants and to set up an 

 impassable barrier for many of them ; and secondly, that many plants have the 

 capacity of adapting themselves to various degrees of light intensity; but in conse- 

 quence of this they occasionally develop such a varying character that they might 

 be mistaken for wholly different species. But I shall return again later when 

 speaking of the origin of new species to this result of cultivation. Here we shall 

 only discuss, in order to prove and make clear the connection between certain plant 

 characteristics and the conditions of illumination, how it happens that the surface 

 of foliage exposed to the direct rays of the sun is so frequently coloured violet or 

 red, or is completely covered over with hairs, while the leaves of the same species if 

 they have been developed on shady soil in dispersed light are coloured green, and 

 remain almost bare; how it happens that plants of one and the same species in the 

 deep valleys possess but few hairs, or are provided with but thin cuticular layers 

 but on the sunny slopes of high mountains are shrouded in thick grey or white fur, 

 or appear thick and almost leathery in consequence of strongly-developed cuticular 

 layers. In order to prevent misconception, it must indeed be pointed out here that 

 all this only refers to the epidermis over the green tissue which is exposed to direct 

 or difiuse sunlight, chiefly, therefore, to the upper side of the foliage-leaf, and that 

 when the blue colouring-matter and also the covering hairs are developed on the 

 under side of the leaf, or in floral leaves devoid of chlorophyll, they have then an 

 essentially different significance, which will be described in the next section. 



When describing the protective measures of the green tissues against the 

 dangers of over-transpiration, the vertical direction of branches, flattened shoots, 

 phyllodes, and especially of the green leaf-surfaces, was pointed out. The leaves of 

 irises, and of the so-called compass-plants, the flattened outspread petioles, with 

 their edge directed towards the zenith, in so many Australian trees and shrubs, 

 were there more especially described, and finally it was pointed out that the 

 leaflets of many papilionaceous plants, and the leaves of numerous grasses, 

 temporarily take up a position by sinking, rising, and folding together, in which 

 not the broad side, but the narrow edge, is exposed to the vertical rays of the 

 mid-day sun. 



A leaf-surface which assumes one of these positions with regard to the sun 

 will transpire much less than a foliage-leaf on whose broad surface the mid-day 

 sun falls vertically, or almost vertically; but by such a position the leaf is also 

 afforded a protection against the too vivid light of noon. The rays which reach a 

 vertical leaf-surface at morning and evening are not so intense as to be able to 

 destroy chlorophyll; they have rather just that intensity which the chlorophyll- 

 granules require for their activity. Therefore, by this arrangement the function of 

 the chlorophyll-granules is not restricted, but is actually assisted, and in this sense 

 the vertical direction of the green surfaces is to be looked upon also as an 

 arrangement for regulating the activity of the chlorophyll-granules. 



It is evident after this explanation that herbs with vertically-directed leaf- 



