GALLS 385 



imbricate, pedunculate, pilose, pubescent, pustulate, rugose, rosaceous, 

 scabious, separate, sessile, solitary, spiny, rolling and thickening of the 

 leaf, upon the upper surface of the leaf, upon the under surface of the leaf, 

 upon the margins of the leaf. Some cecidologists would classify galls 

 by the causal animal or fungus, by the natural families of the host 

 plants, according to the situation of the galls upon the plant, according 

 to their modes of growth, etc. Anton Kerner in his " Natural History 

 of Plants " (translated from the German by F. W. Oliver) divides galls 

 into simple, where one plant organ is involved, and compound, where 

 several plant organs are concerned in their formation. The simple 

 galls he divides into (a) felt galls, (b) mantle galls and (c) solid, or 

 tubicular galls. 



Cataplasmic galls are often produced by the action of parasitic 

 fungi, which invade the interior of the plant after an infection by ani- 

 mals, which by their wanderings over the surface of the plant may en- 

 large the field of their stimulation. Domiciled organisms are the cause 

 of prosoplasms, where the extent of the field of stimulation remains the 

 same under all circumstances, and is effective only in certain phases of 

 the development of the host plants. 



The etiology of galls is of great interest. Malpighi in his " Anatome 

 Plantarum" published in 1675-79 attributes the formation of insect 

 galls to the action of a poison excreted by the gall insect. Darwin and 

 Hofmeister explained galls, as the action of different kinds of poisons. 

 The stimuli, which cause the formation of galls, is undoubtedly chemic, 

 some unknown substances excreted by the causal parasite, excite 

 the cells of the host plant to growth and cell division and to different 

 kinds of differentiation. We know nothing definite about the chemic 

 substance, nor have the attempts to produce artificial galls been suc- 

 cessful. Traumatic stimuli, too, must come into play, for injury to the 

 plant goes hand in hand with infection, for the first stage of the develop- 

 ment of galls resembles callous tissue. The galls produced may be due 

 to plants, phytocecidia, or to animals, zoocecidia. The fungi and a few 

 flowering plants are largely responsible, while dipterous and hymenop- 

 terous insects and mites are gall-producing animals. 



(a) Cataplasms. — Cataplasmic galls are those which are distin- 

 guished from the normal tissue of the corresponding organs by the small 

 amount of their tissue differentiation. The cell elements may often be 

 abnormally large, and the union of these elements usually forms a thin- 

 25 



