INSECTS AND PLANTS 



87 



two years ; but, in exceptional circumstances, all the stages 

 may be completed in a single year. Possibly, too, the 

 young insects do not all await the approach of winter 

 before passing to the roots, but continue to do so through- 

 out the summer. One fact, however, has been definitely 

 proved, that the young from the leaf galls may easily be 

 colonised on the roots ; but the process is not a reversible 

 one, for the insects have never been observed to pass from 

 the roots to the leaves. 



