6o BRITISH FUNGI 



The above experiment proves that even when the germs of a 

 disease are present, their further development depends to a very 

 great extent on tlie conditions under which the host-plant is placed. 

 This is a point perfectly well known to the growers of potatoes, 

 although unfortunately they are unable to modify the conditions. 

 The occurrence of a few cloudy, damp, warm days spells potato 

 disease to the farmer, such conditions being unfavourable for the 

 growth of the potato plant, but highh^ favourable for the parasite 

 in its tissues. In all probability external conditions determine, 

 to a far greater extent than we at present recognize, matters 

 relating to health or disease in plants. The same is true of the 

 animal kingdom. A person tainted with pulmonary trouble, by 

 living in a dr\', bracing climate, can hold the disease in check, 

 whereas if living in a damp climate the disease usually predominates. 



To return to the progress made in the art of parasitism by the 

 potato fungus. From the above account it will be seen that the 

 higher, sexual, or winter-spore stage has completely dropped out, 

 its function of continuing the species from year to year being re- 

 placed by the development of permanent or hibernating mycelium 

 in the tubers of the host-plant. The conidial or summer-spore 

 form of fruit has been retained, by which means it is enabled to 

 extend its geographical area. Thus there are two distinct methods 

 b}' which the potato disease fungus can be spread, (i) By planting 

 tubers containing the mjxelium of the fungus, in a district pre- 

 viously free from disease. By this method the potato disease has 

 been spread in a wholesale manner, and it is now present in every 

 part of the world where the potato is cultivated. (2) By means of 

 summer-spores, which are dispersed by various agents, and con- 

 tinually infect plants previously free from disease. 



A yet more highly developed type of parasitism, where the health 

 of the host-plant is not in any way interfered with, and where spore 

 formation of every kind has been dispensed with, the only method 

 of propagation being assured by hibernating or perennial mycelium 

 is met with in the fungus parasitic in the grain of certain kinds of 

 rye-grass, as Lolium perenne, L. tcnmlentiim and L. italiciini. The 

 mycelium of the fungus is present in the " seed " or grain. On the 

 germination of the grain the mycelium assumes active growth, and 

 keeps pace with the growing stem of the grass, living in the tissues, 

 and continues to do so until it enters the new seed, where it remains 

 along with the seed, in a resting condition until the seed commences 

 to germinate, when the same cycle of growth is repeated. The 

 presence of the fungus in the tissues during growth in no way inter- 

 feres with the functions of the host-plant ; in fact, it has been proved 

 that infected plants are more vigorous and robust than uninfected 

 plants. So complete is the mutualism between host-plant and 

 fungus, and so certain is the latter of perpetuating itself by mycelium, 

 that the production of spores or fruit of any kind has been arrested. 



