SAPROPHYTISM AND SYMBIOSIS 



781 



tural modification of a tissue or of an organ due to another organism. 1 

 Commonly the affected tissue is much enlarged, either through hyper- 



f 



FIG. 1089. * A cross section through the edge of a leaf gall of Viburnum Lantana, 

 showing striking hyperplasy and hypertrophy; p, the palisade cells of the uninfected 

 portion ; p', the corresponding cells of the infected portion ; /, the sponge cells of the 

 uninfected portion; /', the corresponding cells of the infected portion; e, epidermis; 

 h, epidermal hair; considerably magnified. From KtisxER. 



trophy (cell enlargement) or hyperplasy (development of new cells), 

 or through both combined (fig. 1089). Scarcely less common is the 



1 Galls also are termed cecidia and have been contrasted with domatia, which differ 

 in that the organisms inhabiting them are thought to be harmless or even beneficial 

 rather than detrimental to their hosts. An example of domatia is afforded by the root 

 tubercles of clover; sometimes the structures inhabited by plant lice are regarded as 

 domatia, since the nitrogenous animal excreta are thought to benefit the plant. It seems 

 wiser to call all such structures galls, regardless of their benefit or harm to the host plant. 



