24 THE MORPHOLOGICAL DISTRIBUTION [CH. 



very readily produce anthocyanin. On the other hand, the majority 

 of the trees in the Amentales do not form red pigment in the autumn 

 (species of Salix, Populus, Betula, Alnus, Juglans, Morus, Carpinus, 

 Corylus, Castanea, Fagus and Quercus). To the above, Quercus rubra 

 and Q. coccinea form an exception. On the whole, however, little 

 emphasis can be laid on the connection between autumnal coloration 

 and relationship, as is illustrated by the Viburnums V. Opulus and 

 V. Lantana ; the former rapidly reddens in autumn whereas the latter 

 forms little or no anthocyanin under similar conditions. 



5. In a number of species, mechanical injury frequently leads to 

 formation of anthocyanin in the portion of leaf, stem, or shoot, distal 

 to the point of injury. Partial severing of a portion of the leaf blade 

 may induce reddening in the severed portion. Crushing, or partial 

 breaking of a stem or petiole, leads to the same result, as was shown 

 by Gautier (175) in the Vine, or ringing of a stem will cause the leaves 

 above to turn red, as Combes (374, 385) has demonstrated in Spiraea. 

 The same kind of injury, with similar results, may be brought about 

 by the attacks of insects. Under this heading must be included, as a 

 result of injury, the copious formation of anthocyanin in galls, such as 

 those which occur on species of Rosa, Salix, Quercus and many other 

 plants. Also the local appearance of pigment in tissues infected with 

 Fungi : as an example one may quote the purple blotches produced 

 on leaves of Tussilago Farfara and the purple streaks on Wheat stems 

 when these plants are infected with Puccinia. The physiological causes 

 involved in the production of pigment on injury are dealt with in 

 Chapter vi. Some plants show a greater tendency to redden on injury 

 than others (Oenothera, Rumex, Rheum) ; also the reddening on injury 

 is more rapid and frequent in autumn when the vegetative season is 

 about at an end. 



6. Low temperature may induce anthocyanin formation apart from 

 autumnal coloration, for, in the development of the latter, factors 

 other than temperature play an important part. Many evergreen 

 shrubs show a reddening 1 in their leaves during winter (Ligustrum 

 vulgare, Mahonia spp.). The same statement holds good for certain 

 herbaceous plants which retain their leaves in winter (Saxifraga umbrosa). 



7. Plants exposed to drought often develop anthocyanin. This 

 may be seen sometimes in individuals in pots (Pelargonium] which have 



1 This is of course quite different from the reddening shown by some evergreens in 

 winter, notably species of the Gymnosperms, and which is due to the formation of a red 

 product from chlorophyll in the chloroplastids. 



