ON THE ORIGIN OF THE FORMS OF GALLS. 31 



mentioned secretion, nor the wound made by the 

 ovipositor when the eggs are laid, has anything to 

 do with the growth and formation of the gall.* It 

 may be t^vo or three weeks, or tAvo or three months 

 after the eggs were laid before the gall commences 

 to groTv; it then only does so when the larva has 

 left the egg and has begun to feed, the formation 

 of the gall being synchronous with the birth of the 

 larva. Thus it is evident that the irritation caused 

 by the feeding and growth of the larva must be 

 regarded as the main, if not the exclusive factor, in 

 the formation of the galls of Cyiupida:. 



Before any gall-groT\^th can take place it is 

 absolutely necessary that the eggs should be de- 

 posited in the cambium-layer of the plant. It is 

 easy to see why this should be so ; for the cambium- 

 layer is the seat of active growth in the i3lant ; it 

 is composed of formative, not of formed matter, 

 and hence we can readily perceive why it so sus- 

 ceptible of stimulation from foreign influence. The 

 more active the growth in the cambium-layer it 

 might be expected that it would be the more 

 susceptible to external irritation, and that the 

 growth of the galls would be most rai)id when the 

 layer itself was at its most active i^eriod. This is 

 precisely what we find to be the case. The sirring 

 galls reach maturity and even decay in two or 

 three weeks; eggs laid in the leaves in summer 

 take much longer- some two or three months — to 

 become fully grown ; and still slower is the 

 development of the bark and root galls. 



The action of the larvae on the galls is well 

 shown by the inquiline flies. These lay their eggs 

 n the galls when they are quite young ; their 

 larvte are quicker in growth than those of the 

 gall-makers, and they soon monopolise the gall, 

 distorting it, and often converting a monotlialamous 

 nto a polythalamous gall. One of the best 



* It must hi understood that this paper deals only with the 

 galls of the Cy nip idee. 



