CANKER OF THE APPLE TREE. 575 



gall-producing Aphides, Laclmus, and Schizoneura. The 

 common aphides of the ivy and the rose prick and suck the 

 soft young stems, but there is no specific reaction on the part 

 of the plant such as we see in the case under consideration. 



The natural history of the Schizoneura is as follows. Dur- 

 ing the winter months the mature insects find shelter in the 

 cavities and crannies of the nodosities, and in the early days 

 of spring their presence is noticeable from the patches of 

 white fluff. These inci'ease in size owing to growth in number 

 of the insects, and as the summer advances masses of sticky 

 fluff envelope the branches in which ai-e found crowds of 

 aphides. Some of these become '' nurses " and produce living- 

 young parthenogenetically. A new generation is said to be 

 produced every fourteen days, and as the young twigs of the 

 tree grow new colonies are founded. The place where the 

 colony is started on the twig of the year does not appear to 

 be a matter of chance but rather of selection. At the time 

 that the twigs are invaded the young leaves are well formed, 

 the distal end of the stalk is green, but towards the parent 

 tree the green passes into a reddish-brown colour indicating 

 the formation of a periderm, which, in the case of the apple, 

 is derived from the epidermis. The place of selection is not 

 the green portion of the twig at the extremity, but nearer to 

 the old wood at a point where the wood has definitely formed 

 and the formation of the periderm commenced. Some of the 

 "nurses^' descend to the roots as in the Phylloxera and 

 establish colonies there producing deformities similar to those 

 on the stem. 



If a tumour is selected for study on which the aphides are 

 actively feeding, and after fixing, hardening, etc., sections 

 are prepared and stained in a manner to differentiate the 

 tissue, such as by iron haematoxylin method and fuchsin, the 

 appearance represented by fig. 3 will be seen. 



On the outside is a layer of cells two or three in thickness, 

 which is the periderm, beneath this is the cortex with strands 

 of sclerenchyma. If we follow the cortex over the tumour 

 we shall see that it has undergone slight alteration only, a 



