1916-17.] BOTANICAL SOCIETY OF EDINBURGH 225 



old, show as yet any of the thick butt' indumentum of 

 the adult. 



The illustrations I have named show that the develop- 

 ment of anthocyanin pigment in the leaves of the juvenile 

 state is associated with varied forms of hairy indumentum 

 in the leaves of the adult. The history of the transition in 

 the several species remains to be traced. In many species 

 scattered sebaceous floccose hairs appear on the veins before 

 the coating becomes a true tomentum. I may add that in 

 Mh. Antho'pogon, David Don, in which the adult leaves have 

 a lepidote indumentum, there is reddening of the under- 

 surface in the juvenile leaves. 



The facts suggest that there is here a change of con- 

 struction in relation to a change in climatic relation. The 

 plantlet passes from a position in which its functioning 

 foliage is subject to all the conditions of light, moisture, 

 heat, and air-current, belonging to a stratum at the soil- 

 surface, to one some distance above the soil-surface, in 

 which the same external influencing factors operate in 

 different co-ordination and intensity. Temperature and 

 speeding up of metabolism are prime considerations in the 

 one environment, control of transpiration in the other. 

 The anthocyanin development is an adaptation to the 

 former, the indumentum to the latter. Material devoted 

 to the making of relatively unstable cell-pigments in the 

 early phases of ontogeny is now used for the building of 

 tissues — what a complex laboratory it is ! — and perhaps 

 there is special significance in the fact of the indumentum- 

 formation so often beginning at and about the midrib and 

 leaf -base. It certainly secures first attention to the forma- 

 tion of the indumentum hairs. 



This anthocyanin formation on the undersurface of 

 leaves is not unknown elsewhere. Text-books record it 

 particularly in plants of woods and like-shaded areas, and 

 observers have pointed out that the coloration is rare in 

 plants which are woolly or have otherwise constructed 

 hair-coverings. Here, in those Rhododendron seedlings, we 

 have the states combined, and the seedlings appear to offer 

 particularly favourable objects for experimental work bear- 

 ing upon the functions performed by anthocyanin pigments. 



