578 JAMES E. BLOMFIELD. 



their nourishment, till large gnarled nodosities are produced 

 consisting of dead, necrosed wood, hypertropliied tumours, 

 and wound wood, which may attain the size of a man^s fist. 

 It is not necessary that the tumour be split for Nectria to 

 gain entrance. In comparatively young tumours the fungus 

 and its necrosing action may be seen, though there is no 

 breach of the surface except the punctures made by the 

 insect, and it is, no doubt, by these that the germ gains 

 entrance. 



To shortly resume the origin of these tumours, we have 

 seen that they are produced by the pricks of the aphides. 

 That during this process some influence is brought to bear 

 on the active cambial cells which leads to their enlargement 

 and increase. That the cells are arrested in their normal 

 development and destiny, and that as long as this influence 

 lasts they serve the purposes of their parasitic victors, that, 

 when these retire, they are able again to pursue their deve- 

 lopment and destiny, but in such a way that the traces of 

 their experiences are not obliterated. 



I have sketched out this view in the diagrams (fig. 7). 



What is the agent of this influence which the Schizoneura 

 is able to exercise on the cambium of the young twig ? As I 

 noted above, mechanical irritation we must dismiss as a cause, 

 and we can only fall back on the hypothesis of a ferment, 

 such as Beyernick suggested under the term growth enzyme. 

 This may come fi'om the salivary glands of the Aphis. 



I have tried to test this question by acting on a suggestion 

 made by Prof. Farmer of inserting a glycerine and water 

 extract of the insects by means of capillary tubes as near the 

 cambium as possible in such a way that the liquid would 

 constantly bathe the cells. No success has followed these 

 attempts. The slight reaction visible at the point of insertion 

 did not amount to more than that produced by a fine wire 

 inserted in a similar way. 



