1916-17.] BOTANICAL SOCIETY OF EDINBURGH 227 



the light-screen hypothesis. The two activities are not 

 incompatible. Experiment has shown that reddened leaves 

 have a higher temperature than green leaves. This surely 

 means greater protoplasmic activity in all directions ; and 

 whilst to some observers the promotion of transpiration is 

 the primary value to the plant of a heat-relation, to others 

 it is the increased metabolism itself. Is there any reason 

 for disallowing either effect at the several times which 

 favour the respective functions ? On the other hand, to 

 some observers the heat-relation of the anthocyanin is 

 solely that of a heat-screen. 



The circumstances of our Rhododendron seedlings seem 

 to point to this heat-activity of anthocj^'aniu as important 

 in their case, for light-poverty and radiation-cold are factors 

 not conducive to copious food-formation, and the profuse 

 root-system and large hydathodes of the seedlings suggest 

 a very free water-current. Heat-accession through antho- 

 cyanin may well be an aid here. 



Not all Rhododendron seedlings show the red pigmen- 

 tation of the undersurface in juvenile leaves. Rh. auricu- 

 latutn, Franch. is a species in which the adult leaves 

 liave a loose underleaf tomentum, and I do not find in 

 the juvenile leaves the red coloration. In species, too, 

 like Rh. glaucum, Hook, f., Rh. hippoi^haeoides, Balf. f. et 

 W. W. Sm., Rh. oleifolium, Franch., and others, where the 

 adult leaves are lepidote and covered beneath by a wax 

 coating giving them a white or bright grey colour, I have 

 not seen the pigmentation^ — the leaves have a wax coating 

 through the juvenile life of the plant. 



On the other hand, the anthocyanin appears in juvenile 

 leaves of species which do not develop on the leaves of the 

 adult any marked indumentum. Species of the Thomsoni 

 Series — using that term comprehensively to include the 

 Campylocarpura Series and the Selense Series — show this 

 markedly. In them there is a well-developed layer of 

 epidermal papillae forming wax. 



These variations point to the value of the seedlings of 

 Rhododendron for a comparative physiological investigation 

 which I believe would throw much light upon the much- 

 discussed problem — the uses of anthocyanin pigments.^ 

 1 See Wheldale, The Anthocyanin Pigments of Plants. 



