520 SOURCES OF HEAT. TRANSFORMATION OF LIGHT INTO HEAT. 



leaf is given in figure 25a, q. Amongst other species belonging to this group 

 may be mentioned Cyclamen repandwm and C. hederifolium, Cardamine trifolia, 

 Soldanella montana, Hepatica triloba, and Saxifraga Geum and cuneifolia. 

 Growing in habitats similar to these are to be met biennial, occasionally 

 perennial, plants which in autumn form a rosette of leaves on their erect stems 

 which survive the winter; these are always coloured violet on the side turned 

 towards the ground, while the leaves which develop in the following warm summer 

 on the elongated flower-stalks usually appear green below. To this group belong, 

 especially, numerous Cruciferse (e.g. Peltaria alliacea, Turritis glabra, Arabia 

 brassicceforrais); species of spurge {e.g. Euphorbia amygdaloides), bell-flowers 

 (e.g. Campanula persicifolia), and hawk weeds (e.g. Rieracium tenuifolium). 

 Finally, deciduous shrubs are to be found in the depths and on the margins of 

 forests whose leaves do not survive the winter, but which produce on the stems 

 developing in the summer flat leaves whose under side contains abundant antho- 

 cyanin, as, for example, Senecio nemorensis and nebrodensis, Valeriana montana 

 and tripteris, Epilobium montanum, Lactuca muralis, and many others. 

 Amongst non-European species may be noticed many Flowering Eushes, Trades- 

 cantias. Begonias, and Cypripediums, as well as the Japanese Saxifrages (Saxi- 

 fraga sarmentosa and cortuscefolia), which are coloured deep violet on the lower 

 side of the leaf with anthocyanin, and are only found in shady spots in forests. 



Since anthocyanin has been already indicated as one of the means of protecting 

 chlorophyll, the question must first of all be considered as to whether such a 

 relation does not exist in the instances just enumerated. It might even be 

 possible that the violet side of the foliage-leaves now turned earthwards was 

 originally turned towards the incident rays of light, while the leaves were still 

 very young, and that the anthocyanin remains in the position once assumed in 

 consequence of the twisting of the leaves, without being assigned any particular 

 function on that account. Opposed to this idea, however, are the facts that in 

 the majority of the plants cited, anthocyanin is only first developed when the 

 side of the leaf in question has already been turned towards the ground; that in 

 many species the violet side is never turned upwards at any period of develop- 

 ment; and especially that in all these plants which grow in the shade, no protec- 

 tion of chlorophyll against an over-abundance of light appears necessary; that, 

 on the contrary, it is important for these shaded growths that the scanty light 

 and heat should be appropriated and utilized to the utmost extent. 



We cannot therefore assign to the anthocyanin on the under side of foliage- 

 leaves any protective influence upon chlorophyll. On the other hand, everything 

 goes to show that the anthocyanin developed here absorbs light and changes 

 it into heat. Light which, passing through the leaf, would reach fallen dead 

 and dry foliage, or the ground itself in the depth of the forest, would be wasted 

 and useless there. When absorbed by the anthocyanin and changed into heat, it 

 becomes serviceable to the plants, and can exert a helpful influence on the growth 

 of neighbouring cells, and to a less extent apparently also on the metabolism and 



