a Note on the early stages of its development. 19 



The only possible explanation of this posthumous growth appears 

 to me to be, that the poison injected during that week of the 

 insect's life continues to act on the tissues of the plant after the 

 death of the mother. 



But this naturally gives rise to another question. If the 

 insects of one generation inject some fluid into the plant, surely 

 the insects of the other generations, the Alatae, the Colonici, the 

 Sexuparae, etc., must also be supposed to inject a poison when 

 they are engaged in sucking. How then is it that they do not 

 cause the formation of a gall ? Why does the injection answer in 

 one case and not in another even on the same tree ? 



The answer to this is, I think, that the injection will only give 

 rise to gall-formation when it acts, not only on embryonic tissues, 

 but on embryonic tissues which are not confined by the presence 

 of other lignified or cuticularised tissues. We have seen how such 

 feebly lignified tissues as the protoxylem strands were sufficient 

 to turn the course of gall-formation from concentric to excentric. 

 Again, when the gall emerges from the bud-scales the gall area is 

 at once confined to the region already affected in consequence 

 of the exposure to light and air, which results in the hardening of 

 all parts which still remain normal. 



The behaviour of the Larch needles when attacked by the 

 larvae of Generation IV. also confirms this. The young needles 

 when they first emerge from the buds are exceedingly tender, and 

 if attacked at this stage, they at once respond to the influence. 

 Each needle becomes bent into the shape of a knee at the point 

 where the insect is seated, and very frequently the "knee" is 

 distinctly swollen, shewing that the tissue has responded to a 

 slight extent and formed a minute gall. The needles however 

 quickly harden under the influence of light and the gall developes 

 no further. When the needles are older and harder the attacks 

 produce neither bending nor swelling, but the chlorophyll still 

 responds in exactly the same way as it does in the Spruce gall. 

 It disappears, leaving a yellow spot to mark each place where an 

 insect has been seated. When old Spruce needles are attacked 

 by the Alatae, the Sexuparae or the Sexuales, the same thing is 

 apparent, and I have little doubt that the injection is the same 

 in all cases, but that the hardening of the surrounding tissues 

 prevents the formation of a gall. 



2—2 



