376 Scientific Proceedings, Royal Dublin Society. 



considerably before the latter show signs of infection. Eain or dew would 

 easily enable such conidia to be carried downwards, both to the cotyledons 

 and the stems of the seedlings. Further, it has been observed that if muci- 

 lage from the infected seeds be placed on the cotyledons of healthy seedlings, 

 conidia are developed from the mycelium some considerable time before the 

 cotyledons show signs of infection. The matter is an interesting one biolo- 

 gically, and may be of some significance from the practical point of view. 

 For, if infection occurs only through conidia, it might be possible to prevent 

 their development (by suitable treatment of the seed) until the seed-coats 

 had been thrown off, whereas the more drastic treatment which would pro- 

 bably be necessary to kill the hibernating mycelium might have an injurious 

 effect on the seedling, or even kill the seed itself. 



VIII. — Control of the Disease. 



Since the disease is transmitted through infected seed, the obvious 1 control 

 measures that suggest themselves are : — 



(1) Prevention of seed infection, and, failing that, 



(2) Disinfection of infected seed. 



We have not yet had an opportunity of ascertaining the exact way in 

 which the seed becomes infected under natural circumstances. It lias already 

 been stated that Collctotrichum linicolum attacks the seed-bolls ; and it is 

 possible that its mycelium passes through the tissues of the fruit-wall, and 

 thus reaches the surfaces of the seeds and infects them. This is apparently 

 what occurs with the allied species, C. Lindemuthicmum (the cause of bean- 

 anthracnose), although in this case the fungus passes through not only the 

 fruit-wall but also through the seed-coat, and thus infects the embryo. 



On the other hand, it may be, as has also already been suggested, that the 

 seeds become contaminated with conidia which, under favourable conditions 

 of moisture, germinate and send their germ- tubes into the epidermis of the 

 seed-coat, there to develop into the hibernating mycelium. It was proved 

 by experiment that infection of the seed by this latter means is at any rate 



1 The close connexion between the incidence of infection and the carrying above 

 ground of the seed-coats on the cotyledons has already been emphasized, and it is con- 

 ceivable that the disease could be obviated by deep sowing. But it has been shown that 

 a proportion of seedlings become diseased even when the seed-coats are not carried up, 

 and these would under favourable conditions act as centres for subsequent infection of 

 neighbouring plants. Further, deep sowing retards germination and lessens the vigour 

 of the resulting seedlings, which is a serious disadvantage ; and in Ireland flax-seed is 

 always sown broadcast and is only lightly harrowed in. Hence deep sowing cannot be 

 looked upon as a suitable means of controlling the disease. 



