SOURCES OF HEAT. TRANSFORMATION OF LIGHT INTO HEAT. 521 



transportations of the substances. In the evergreen leaves of those plants in the 

 depths of the forest which are natives of inclement regions, this advantage is 

 obtained from the layer of anthocyanin developed on the lower leaf -surface, that 

 every sunbeam, even in the cooler seasons, can be utilized to the utmost. It is 

 in harmony with this explanation that foliage-leaves of trees, shrubs, and high 

 bushes which grow a considerable distance above the ground, and have below 

 them other green foliage-leaves, are never violet-coloured on their earthward side, 

 and that in richly-leaved bushes whose lowest leaves lie on the soil, these only 

 are provided with anthocyanin. That portion of the light not turned to account 

 in the highest green leaves, and which is allowed to pass through them, can still 

 be utilized by the lower ones; only that light which would pass through the 

 lowest leaves would be lost to the plants, and therefore we have a violet absorbent 

 layer only on that side which lies on the ground. 



That which occurs in plants of the forest shade occurs similarly in those marsh 

 plants whose leaf-like stems or ilat, disc-like leaves float on the surface of the 

 water. The green discs of duckweeds (e.g. Lemna polyrrhiza), of the Frogbit 

 (Hydrocharis morsus-rance), of the Villarsia (Villarsia nymphoides), of water 

 lilies {Nymphcea Lottos and thermalis), and of the magnificent Victoria regia, 

 are strikingly bi-coloured, being light-green above and deep violet below. Here 

 again it cannot be said that the anthocyanin forms a protection for chlorophyll, 

 but the violet colouring-matter can retain light in the cells on the lower surface 

 of the leaf, and can change it into heat and so make it useful to the plants. The 

 rays which penetrate the green leaf-discs and shine through the water would 

 otherwise be lost to the plants in question, for none of the species enumerated 

 have submerged leaves, but possess only these floating discs, green on the upper 

 and violet on the lower side. 



If anthocyanin were found, not only on the imder but also on the upper side 

 of the foUage-leaves, then indeed the significance would primarily be assigned to 

 it of a means of protection for chlorophyll, and of assisting the metabolism and 

 transport of materials; but obviously the blue colouring-matter would not, on the 

 upper side of the leaf, behave essentially otherwise as regards its capacity of 

 changing light into heat, than on the lower side. It is even probable that the 

 importance of anthocyanin lies, not only in its retention of the rays injurious to 

 metabolism, but also in the transformation of light waves into heat. In support 

 of this view there is at least the fact that anthocyanin is also richly deposited 

 on the upper side of the foliage-leaves at times when, and in places where, other 

 sources of heat are deficient, and that generally leaves and stems of many plants 

 growing in such places are entirely overspread with red or violet. A number of 

 small annuals which grow very early in the spring at a low temperature (e.g. Saxi- 

 fraga tridactylites, Hutchinsia petroea, Veronica prcecox, and Androsace maxima) 

 are usually coloured with anthocyanin on all sides of their growing organs. More- 

 over, seedlings which spring up from the earth at low temperatures, and above 

 aU high Alpine forms in the neighbourhood of the snow -line, are abundantly 



