CANKER OF THE APPLE TREE. 577 



small quantity of coagulated plasma may be found in each 

 cell (fig. 4). 



If the cambial region of the tumour is examined with a 

 high power it is evident that this tissue is in a state of great 

 activity. In appropriate sections the tracks left by the setJB 

 of the insect may be traced through the periderm and cortex 

 till they terminate in the cambium^ and it is around these 

 terminations that the greatest activity is taking place. The 

 distinction between bast and cambium is made out with 

 difficulty, as both kinds of cells are enlarged, and contain 

 large, well-defined nuclei. In places the division of the 

 nuclei exceeds in rapidity that of the cells, with the result 

 that a multinucleated mass is produced. This cell division 

 no doubt takes place by mitosis, evidence of which I obtained, 

 but the material is difficult to cut with sufficient accuracy for 

 a study of this process. After the cells are produced by the 

 cambium further division is undergone, or multiplication of 

 nuclei may take place without corresponding cell division. 

 The protoplasm exhibits vacuoles, which increase in size till 

 the whole cell consists of a wall of cellulose, a small quantit}' 

 of plasma, with a nucleus and a large quantity of cell sap, 

 consisting chiefly of sugar. 



That these changes are produced by the aphides is shown 

 by the fact that, if they are swept away by wind or rain, the 

 cambium resumes its normal activity, and gives rise to cells, 

 which pursue their destiny of lignification in a normal 

 manner, enclosing a portion of the tumour, which itself 

 undergoes lignification, but, from the displacement, increase 

 in number (hyperplasia) and in size (hypertrophy), the 

 elements are abnormally arranged, and produce a condition 

 which is known as wound wood (fig. 6). 



If the aphides linger on their tumour and the weather is 

 dry the soft parenchymatous tissue may split and allow a 

 genus of destructive fungi such as Nectria to enter, pro- 

 ducing necrosis and ulceration of the tissue. This the plant 

 tries to counteract by its powers of healing, and new cambial 

 tissue is produced, to be quickly utilised by the aphides for 



