SAPROPHYTISM AND SYMBIOSIS 



783 



layers, in which food accumulates abundantly, and later is utilized by 

 the parasite, whether fungus or insect. A somewhat complex situation 

 is found in the cynipid galls (i.e. galls formed by members of the Cyni- 

 pidae, a group of highly specialized insects) ; here the larval chamber is 

 surrounded by well-defined food layers, which sometimes are differentiated 

 into an inner protein layer and an outer carbohydrate layer, the whole 

 being surrounded by a layer of rigid mechanical cells or of protective 

 cork cells (figs. 1090-1093). Sometimes the nutritive layers remain meri- 

 stematic, continuing to regenerate as they are destroyed by the insect 

 larvae. Galls are unusually rich in tannins, resinous secretions, and 

 other waste products, and are remarkably resistant structures; if an 

 infested plant is cut down, the galls often remain fresh and green when 

 the other parts are dead. Not infrequently the tissues of galls are much 

 more xerophytic in structure than are the tissues of uninfected organs. 



Galls occur on all kinds of plant organs and they assume a wide variety of forms, 

 some of which are exceedingly fantastic. They are caused by many species of 

 plants and animals, but chiefly by 

 fungi and insects. Often the organ 

 affected is greatly modified in form, 

 and in some cases structures appear 

 that are not present when the plant 

 is uninfected. For example, the rose 

 gall, formed by the cynipid insect, 

 Rhodites bicolor, is covered with 

 prickles, even if the rest of the plant 

 is quite smooth (figs. 1094-1096); 

 similarly, the grape-vine gall formed 

 by Cecidomyia Vitis-pomum (fig. 

 823), and some cynipid oak galls are 

 pubescent, though the organs con- 

 cerned are smooth when uninfected. 

 In the common oak gall, formed by 

 Amphibolips inanis, delicate threads 

 connect the larval chamber with the 

 gall periphery. Some gall-forming 

 organisms injure the growing point, 

 thus checking elongation, and caus- 



FIGS. 1094-1096. Gall f6rmatipn in a 

 rose (Rosa blanda): 1094, an ordinary leaf 

 with five leaflets (/) and a pair of stipules (s); 

 1095, a leaf which has been attacked by a 

 gall-producing insect (Rhodites bicolor); note 

 the prickly galls (g); 1096, a similar leaf still 



more modified through gall formation. 



ing the close imbrication of the de- 

 veloping leaves, as in the conelike 

 willow gall formed by Cecidomyia strobiloides, and in the goldenrod gall formed 

 by C. Solidaginis (figs. 1097-1099). A great many root galls are formed by 

 nematode worms. 



An interesting group of galls are the witches' brooms, which are formed on various 



