External Factors that Influence Growth 271 
was shown by Adler’s experiments it had been supposed to 
belong to a different genus. The gallflies of Spathegaster are 
both males and females. The males emerge first, and as soon 
as the female appears copulation ensues. If the fertilized fe- 
males are placed on oak saplings having tender young leaves in 
actual growth, these are pricked on the under surface. Galls be- 
gin to develop after two weeks, and remain on the tree until the 
autumn. The fly that emerges from these is Neuroterus len- 
ticularis, which completes the cycle. 
Other species of gallflies have only one generation a year, 
which is parthenogenetic. Others still may have two partheno- 
genetic generations a year, and consequently no sexual reproduc- 
tion at all. There is great diversity in the kinds of galls. In 
fact, the galls often differ more from each other than do the species 
of gallflies that produce them, and are more easily identified. 
Three points in the formation of the galls are especially inter- 
esting. First, the gall develops always from unformed tissue, 
particularly from the meristem of the plant. Second, the kind 
of cells and tissues that form the galls are those peculiar to the 
plant on which the gall develops; but while some of the cells 
may retain to a large extent their original structure, others be- 
come changed. Spiral vessels grow into the gall and ramify 
through its walls in definite courses. Third, while the growth 
begins in the vicinity of the larva, the principal changes may not 
be in this part, but removed some distance from the larva, espe- 
cially in the more complicated forms of galls. There can be 
little doubt that something set free from the larva affects the 
cells and causes their remarkable growth. Furthermore, the 
stimulus must be kept up if the gall is to continue to grow. We 
can easily imagine that very small quantities of the stimulating 
substance, continually applied, are effective in producing the 
change, and we can see, if this is the case, how difficult it may be 
to imitate artificially such a process; yet experiments along these 
lines should be undertaken, for, if gall-like growths could be 
artificially induced, we would have a better means for studying 
the processes involved. 
