82 PHYSIOLOGICAL CONDITIONS AND FACTORS [CH. 



believing that anthocyanin is not readily produced where carbon 

 assimilation is most active, and that decreased photosynthesis from any 

 outside cause is favourable to its formation. 



Hence in any consideration of the direct bearing of outside factors, 

 such as temperature and light, on anthocyanin formation, recognition 

 should be first given to possible indirect effects produced by these factors 

 through the medium of photosynthesis. 



Connection with accumulation of synthetic products. 



Even at the height of summer when the vegetative organs are in 

 a condition of maximum activity, quite a number of individual plants 

 may be found having isolated leaves or shoots which are either entirely 

 red or have developed patches or blotches of anthocyanin. In the 

 majority of cases, one will find on careful examination that there has 

 been some injury to the leaf, petiole or stem, as the case may be, and it 

 is to the distal side of the injured spot that the reddening occurs. Such 

 injuries may be classified as: (1) mechanical, caused by chance cutting 

 or breaking; (2) attacks of insects, including gall insects and cater- 

 pillars; (3) infection by Fungi. 



(1) Let us deal first with mechanical injury. Frequently leaves 

 may be found in which the lamina is partially severed transversely, and 

 the severed portion has reddened. Or the petiole or stem is partially 

 broken, and the leaf or leaves above the point of injury have turned 

 red. In Rumex, Oenothera, Pelargonium, Plantago and many other 

 plants, it is easy to bring about such reddening artificially by pinching 

 the lamina or petiole, and in other cases by decortication. Or sometimes 

 isolated leaves, as for instance those of Rheum, left lying on the ground 

 in a damp place will eventually redden. In other genera and species, it 

 is difficult, or impossible probably, to induce reddening by such means. 

 Reference to these phenomena has often been made by various authors : 

 Gautier (175), Kraus (311), Berthold (64), Linsbauer (341), Kiister 

 (350), Daniel (337) and finally Combes (374, 385). Further Combes has 

 made a series of experiments on decortication of stems of many species 

 with a view to investigating the phenomena more fully. By these means 

 he has distinguished three types of results : 



(a) Those in which anthocyanin appeared more or less rapidly 

 in the branches, petioles and above all in the leaves. Ex. Spiraea spp., 

 Mahonia aquifolium, Prunus Pissardi. 



(6) Those in which anthocyanin appeared more or less rapidly in 



