50 THE LARCH CANKER 
recorded artificial infection with larch canker was per- 
formed by a practical forester named Fischer, who caused 
canker in trees by inserting into a wound made in healthy 
bark a suitably shaped piece of bark and phloem from 
a canker on another tree. This experiment proved that 
canker was due to a transmissible cause and was not merely 
the result of unsuitable growth conditions. That the fungus 
Dasyscypha calycina is actually responsible for canker was 
first proved by Hartig when he induced the disease by 
infection with ascospores of the fungus. An account of his 
experiments has already been given on p. 23, and need 
not be repeated here. : 
Massee (1902) also produced canker by means of artificial 
infection, and obtained results which are important in two 
respects. Firstly he found that canker resulted when 
ascospores were placed on the mucilaginous excretion of 
Chermes abietis in the spring. This he attributed to the 
small holes which the insect makes in forcing its proboscis 
through the cork to the living tissues beneath; and he 
supposed that the fungal germ tubes grew down these holes 
and thus gained admission to the cortex and phloem helow. 
Massee attached great importance to this method of 
infection as a source of canker in nature, and considered 
that if the larch aphis could be destroyed, canker would 
cease to be a serious epidemic disease. As I shall show, 
however, the ‘aphis can only assist in the infection of com- 
paratively young stems, whereas the cankers which are 
important to foresters are mostly those which have been 
induced after the stems are three years old. 
Secondly, Massee found that Dasyscypha calycina is 
capable of infecting other trees besides the European 
larch. From the spores of the fungus he produced canker 
on the Scots pine and on two other species of larch—the 
Siberian (Larix siberica) and the Japanese (Larix leptolepis). 
In my own infection experiments, inoculation was 
obtained by inserting a small piece of agar-agar, with 
fungus mycelium in pure culture, in a slit made with a knife 
in the bark of young trees. Infections made in Munich in 
