90 SYMBIOSIS. 



opening of the flower-buds of unattacked twigs, the normal 

 foliage coming later. 



We have here an expression of the existence of a closer 

 symbiotic relationship between the fungus and its host- branch, 

 than between that host-branch and its main branch. It also 

 shows that the host-branch is completely at the service of the 

 fungus, although the latter is dependent on the former for its 

 support. The host-branch is, at the same time, under the 

 necessity of conducting itself iu the partnership in the way most 

 suitable to the development of the fungus.^ Thus the asci of 

 the Exoasceae are produced on the leaves of the witches' broom, 

 and ripen as the normal leaves unfold, so that the spores 

 are in a position suitable for successful infection of the young 

 normal leaves. 



From these facts it can be deduced, that parts of plants 

 attacked by fungi exhibit that kind of symbiosis with the fungus 

 which we call individuation, the joint community behaving 

 more or less as a parasite on the stem or branches of the host- 

 plant. This is clearly the case where the attacked parts exhibit 

 increased growth, and at the same time a diminished production 

 of chlorophyll resulting from degeneration of chloroplasts. Such 

 parts of plants are quite as individualized as the lichens, with the 

 single distinction that they remain in communication with the 

 parent plant and draw nourishment from it. 



There are, however, other cases where the chloroplasts are 

 apparently increased, where at least they attain a lengthened 

 duration of life. This is evident in certain instances first 

 pointed out by Cornu, mentioned* by De Bary, and on 

 which I have made extended observations. Maples may 

 be found in autumn on whose discoloured, withered leaves 

 large green spots are still present. On Norway maple I have 

 observed these spots, very conspicuous on almost every leaf, 

 and especially on those of the lower crown. The green parts 

 were beset with the white epiphytic mycelium and perithecia 

 of Uncinula aceris. Cornu describes similar appearances accom- 

 panying another Erysiphe, certain Uredineae, and Cladosporium 

 dendriticum. I have seen the same phenomenon regularly on 

 the mountain maple on leaves carrying black spots of Rhytisma 



' Plant-galls caused by animals also exhibit adaptations serviceable only for 

 the gall-occupant. 



