HOW FUNGUS DISEASES ARE DISSEMINATED 21 



and plant attacked is very one-sided ; the parasite is provided 

 with a home, and feeds entirely on those substances which 

 its host-plant had prepared for its own use. Thus the 

 parasite obtains all that it requires at the expense of 

 the host-plant, while the latter derives no benefit what- 

 ever, but otherwise, from its parasite. In some instances, 

 however, the relation between what were in the first instance 

 parasite and host respectively, has become so much modi- 

 fied that the two live together and derive mutual bene- 

 fit from each other's presence. This condition of things 

 has reached its climax in the Lichens. Each lichen is a 

 combination of a fungus and one or more kinds of alga, 

 which are morphologically quite independent of each other, 

 yet fungus and alga respectively do a certain amount of work 

 in connection with the production of food that the other 

 constituent could not perform. This condition of things is 

 known as symbiosis or mutualism. 



Very few fungi and flowering plants have attained to the 

 condition of symbiosis, but a very remarkable instance has 

 been shown to exist between a fungus and certain rye-grasses, 

 Lolium temulentum, L. peren?ie, and L. italicum. In L. 

 temulentum, the life-history has been worked out by Freeman. 

 Briefly, the mycelium of the fungus is located in the 'seed.' 

 On germination this mycelium becomes active and keeps pace 

 with the growing stem of the grass, and continues to do so 

 until it again enters the 'seed,' where it remains in a resting 

 condition until the seed germinates, when the same cycle of 

 growth is repeated. The presence of the fungus in no way 

 interferes with the function of the seed, and experiments 

 showed that infected plants were more vigorous and robust 

 than uninfected ones. So complete is the symbiosis, and so 

 certain is the fungus of perpetuating itself by the vegetative 

 method described, that the production of spores or fruit of 

 any kind has been arrested ; consequently, we have no means 

 of determining with certainty the affinities of the fungus. 

 From this it follows that no infection of other plants of the 

 same kind can occur. We have two distinct races of each of 

 the three grasses : one race infected and always producing 

 infected seed, so again the disease has become constitutional. 

 A second race is free from disease, and without the possibility 

 of becoming infected. A microscopical examination of a 

 commercial sample of the seed of L. temulentum showed that 

 over eighty per cent, were infected, hence the facility for 



