272 Experimental Zoology 



Beyerinck made a careful study of the galls of the willow 

 (Salix amygdalina) produced by the 1 gallfly, Nematus capreae. 

 This case is especially interesting, since the galls begin almost at 

 once to develop, and may be full sized before the larva hatches. 

 The cause of the growth in this case is the albuminous secretion 

 that the gallfly injects along with the egg into the leaf. That 

 the secretion and not the egg is the cause of growth has been 

 shown by puncturing, and thus killing the egg with a fine 

 needle. The gall continued to develop. This gallfly deposits 

 its egg in the young leaves of the willow in the early spring. 

 The gall can be detected within two days and has finished its 

 growth in three weeks. The larva feeds on the inner wall of 

 the gall, and finally bites a hole in one side through which it 

 escapes after the gall falls to the ground. It then spins a cocoon 

 and in August the adult fly, the second generation, emerges. 

 It seeks young growing buds of the willow and pierces them. 

 The gall develops in the autumn and falls to the ground with the 

 leaves. The larva spins its cocoon, inside or outside the gall, 

 and overwinters in this condition. In the first generation, in 

 the spring, there are no males; in the second generation occa- 

 sionally males may be found, but nevertheless Beyerinck thinks 

 that this. generation also reproduces by parthenogenesis. 



Beyerinck has discussed the question of the kind of changes 

 that occur when a gall is produced, and he has carried out some 

 experiments that bear on this important question. He tried 

 to determine whether the cells of the gall are permanently 

 changed, i.e. whether their structure has been so affected that 

 whatever they produce will be different from the tissues of the 

 parent plant from which they arise ; or whether the change is 

 only temporary, depending on the presence of some substance 

 exciting them to a peculiar mode of growth. The latter view 

 seems to him the more probable as shown by the following facts : 

 If the leaves are removed from a twig bearing the so-called 

 "willow- rose" gall, new buds grow out of the axils of the leaves 

 of which the gall is made. The first leaves are somewhat modi- 

 fied, like those of the "willow-rose," but the later leaves are like 



