GALLS. 



Notes on some of the 



GALLS MADE ON PLANTS BY INSECTS 

 AND THEIR RELATIVES 



Plant Galls are interesting to the zoologist because most 

 of them are made by animals; to the botanist because of 

 the unsolved problems of abnormal plant growth they 

 present ; and to all of us, not only because ornamental and 

 useful plants are frequently damaged thereby, but also be- 

 cause much of our food is dependent upon them. Potatoes 

 are fungus root-galls, and the bacterial root-galls of 

 legumes are Nature's principal agents in making atmos- 

 pheric nitrogen available for plant use. Of the galls 

 caused by insects, the only ones of commercial benefit 

 are the oak galls, which have been used in dyeing, tanning, 

 and the manufacture of ink. 



As is the case with so many things in natural history, 

 we must go back to Pliny for the first ideas concerning 

 plant galls. This philosopher knew that a fly was pro- 

 duced in them, but he did not associate this fly with the 

 cause of the gall growth. He thought that galls sprang 

 up in a night and that the fly larvae merely devoured this 

 growth. However, the interest of the early observers 

 was not always entirely biological. Important prophecies 

 were deduced as to the events of the coming year by 

 observing whether galls contained spiders, worms, or flies. 



The constant occurrence of certain larvae within certain 

 galls at length aroused the suspicion that galls were 

 formed by the larva?. To account for the presence of the 

 egg and larvae, it was supposed that the female insect laid 

 the egg in the ground and thence it was drawn up with the 

 sap and carried to the outer parts of the plant, where it 

 lodged and gall formation ensued. This theory soon 

 met with opposition. Redi, a poet and physician of the 

 seventeenth century, not having seen the eggs laid, as- 

 sumed that the plant had a "vegetable soul" which pro- 

 duced galls with their eggs, larvae, etc., while at the same 

 time, it gave birth to flowers, fruits, and seeds. 



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