36 GENERAL BIOLOGY 



fungi, and by a number of other less important and less 

 common agencies. All parts of the plant are subject to 

 these malformations. 



As the raking of a wire against a tree trunk that is swayed 

 back and forth by the wind causes great ridges to grow 

 upon the sides of the trunk, so the gnawing or sucking of an 

 insect in the growing tissue of the plant causes a gall to 

 grow. Not all irritations to plant tissues cause such over- 

 growths, but only such as are applied while the tissue is 

 rapidly developing. There are, for example, a number of 

 moth larvae that work in the stems of goldenrods: those 

 whose attack is made before the stem tissues are fully formed 

 cause galls; the others are merely stem borers. Like- 

 wise, in oak leaves the little fly larvae that attack them 

 in the bud cause galls; the later ones make only leaf 

 mines. The stimulus might be the same, but the period 

 of response on the part of the plant being overpast, there is 

 no gall formation. Overgrowth of the plant tissue is, 

 therefore, the criterion of a gall. 



So generous is the response of the plant in the pro- 

 duction of tissue that serves for both food and i shelter, 

 that the habit of attacking young tissues has been biologi- 

 cally profitable. Hence there is developed a large fauna 

 especially and exclusively adapted for exciting galls and 

 living in them a very favorable subject for the study of 

 interrelations. 



We will confine our study here to those malformations 

 that are caused by insects and mites, notwithstanding 

 that there are some common and conspicuous galls, like 

 the one on the sumach top shown in figure 28, made by 

 fungi. This one belongs to that general class of galls 

 popularly known as "witches' brooms": other common 

 fungus galls appear as knots and swellings upon the trunk 

 or the branches of trees : all consist of more or less solid tissue 



