20 OECOLOGICAL FACTORS AND THEIR ACTION SECT, i 



The epidermis of the heliophytt is thick ; usually contains no chloro- 

 phyll, at least on its upper face ; * sometimes it is converted by periclinal 

 divisions into an aqueous tissue several layers in thickness (e. g. in Ficus 

 elastica and some other tropical plants) ; its cuticle or cuticular 

 layers are thick. The epidermis of the sciophyll is thin, one cell in 

 thickness, sometimes contains chlorophyll, and its cuticle is thin. 

 The heliophyll is often very glossy and a good reflector of light, as is 

 demonstrated by many tropical examples ; z the sciophyll is dull in 

 surface and, when subjected to dry air, fades much more easily than the 

 heliophyll. The epidermal cells of heliophylls have less sinuous lateral 

 walls than those of sciophylls. Stomata of the dorsiventral helio- 

 phyll are confined to the lower face, or are more numerous there than 

 on the upper face (except in some alpine plants), and are often sunk 

 below the level of the surface ; those of the sciophyll are on both faces, 

 but perhaps on the whole more numerous on the lower face, and are 

 inserted at or above the level of the surface. Many tropical sciophytes 

 have velvety leaves that are beset with refractive papillae, which serve 

 to collect the obliquely incident rays of light. 3 



7. Lignified parts are more general, in heliophytes than in sciophytes, 

 for example, the production of thorns is more frequent. Heliophylls are 

 often stiff and coriaceous (sclerophyllous plants), partly from lignification, 

 partly because of their thickness, and partly because of the nature of 

 the epidermis ; sciophylls are thin and, if large, flaccid (many herbs in 

 European forests, such as species of Corydalis and Circaea, Lappa nemo- 

 rosa, Lactuca muralis, Oxalis Acetosella, many ferns ; and in the tropics, 

 Hymenophyllaceae, mosses, and others). 



8. In the production of hairs variety is exhibited. Heliophylls often 

 have a dense covering of hairs, a grey tomentum, a silvery coating, or are 

 hairy in divers ways, especially on the lower face (e. g. many plants on 

 rocks, heaths, and steppes) ; sciophylls are universally much less hairy, 

 sometimes quite glabrous. 



9. In the sensitiveness of chlorophyll to light, great differences probably 

 exist, for presumably the chlorophyll of sciophylls is more sensitive than 

 is that of heliophylls, and is consequently better able to utilize weaker 

 light. This suggestion harmonizes well with the fact that an alcoholic 

 solution of the chlorophyll of sciophylls is very easily decolorized in the 

 presence of light. 



10. Light influences the coloration of plants by its action in regard 

 to the production not only of chlorophyll but also of red cell-sap (antho- 

 cyan or erythrophyll). This pigment occurs especially in young parts of 

 plants (in young shoots and seedlings), in autumn leaves, in alpine 4 and 

 arctic 5 plants, in tropical sciophytes. 6 Engelmann has demonstrated 

 that it absorbs the rays of light complementary to those absorbed by 

 chlorophyll ; red leaves exposed to radiant heat acquire a higher tem- 

 perature than green leaves do. The red pigment provides the means 

 of storing up heat, which is available in connexion with metabolism 

 when the temperature of the air is relatively low. 7 It may also be men- 

 tioned that the colours of leaves, flowers, and fruits become deeper in 



| Stohr, 1879. * Volkens, 1890. 3 Stahl, 1896; Haberlandt, 1905. 



' ei ?. er ' * 7 8 Th " Wulff ' I9 2 ' ' Stah1 ' I8 96- 



Stahl, 1896; Buscahom et Pollacci, 1903; Jonsson, 1903; Overton, 1899. 



