THE LARCH CANKER 29 
three cells round the penetrated area, becomes yellow, and 
cells at a considerably greater distance contain more crystals 
than normally. The contents of the medullary ray paren- 
chyma become brown, and the nuclei appear irregular, 
through two or three annual rings of xylem before the 
hyphae can be observed to have reached the cambium. 
There is thus evidence that some poisonous substance is 
transmitted from the hyphae to surrounding cells (especially 
along the medullary rays), which first stimulates excessive 
respiration in the cells, and ultimately causes death. It is 
not clear whether this substance is actually secreted by 
the hyphae, or whcther it is some product of decomposition 
of the attacked cells, but there are certain reasons for 
favouring the former view. A phenomenon which is of 
interest in this connexion was observed in artificia] cultures 
of Dasyscypha calycina on nutrient agar.’ Streak cultures 
were grown in Petri dishes, and all round the mycelium 
a kind of halo was noticed, which proved to be due to 
numerous crystals of monohydrated calcium oxalate extend- 
ing to a distance of 1 cm. from the mycelium. Presumably 
oxalic acid was secreted by the hyphae, and was converted 
into calcium oxalate by the calcium in the substratum. 
This phenomenon is not confined to cultures of Dasyscypha 
calycina ; but it has a particular interest in this case, as it 
suggests that oxalic. acid may be the actual substance 
secreted by the fungus which kills the cells of the host ; 
for probably the same process takes place in the tissues of 
the larch. At first the cells in the neighbourhood of the 
hyphae are able to render the acid harmless by means of 
the calcium at their disposal, which accounts for the accumu- 
lation of calcium oxalate in the affected tissues. But when 
this is used up the concentration of acid gradually kills the 
cells. By this means the fungus is able to spread through 
living tissues without being a true parasite ; that is to say, 
it kills the cells by secretions and enters them when they 
are dead. This kind of false parasitism has been investigated 
1 Agar-agar 10 grm., malt extract 30 grm., meat extract 3 grm., and 
citric acid 0-3 erm. in 1,000 ¢.c. distilled water. 
