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 whether 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 artificial cultures 

 of Dasyscypha calycina on nutrient agar. 1 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 grm. in 1,000 c.c. distilled water. 



