GENERAL ACCOUNT OF FUNGI. 427 



as distinct species, are now recognised as being merely stages 

 in the development of one species. The most highly organised 

 of these stages, in the case of any species, is considered to be 

 that which bears sexually fructifying organs or their equiva- 

 lents. Fungi which thus grow in different forms on different 

 hosts are termed heteroecious ; some, however, go through all 

 their forms on the same host. 



4. Distribution of Fungi. 



The spread of fungi is favoured by certain conditions of the 

 weather and locality. It depends chiefly on heat and moisture, 

 light being prejudicial, many fungi growing in the interior of 

 trees or in the ground. Fungi can flourish and become 

 numerous only under suitable conditions, so that in damp 

 years, and especially in damp sheltered localities, they thrive 

 better than in dry years and exposed places. Thus, in a wet 

 June, Melampsora pinitorqua, Rostrup., is most destructive to 

 pine trees. Nutriment rich in nitrogen favours their growth. 



Parasites attack not only weakly plants but the most 

 flourishing individuals. The conditions which most favour 

 their spread damp air and wet soil are, however, unfavour- 

 able to many woody species, and external injuries of any kind 

 to trees admit the spores of fungi to the inner tissues of the 

 injured tree and thus favour fungoidal attacks. Wild plants, 

 especially when growing in masses, are just as exposed to their 

 ravages as cultivated plants. Coniferae suffer more from fungi 

 than broa^leaved species, because the latter recover more 

 readily from injuries. 



The question, whether a fungus is the cause or conse- 

 quence of a disease can be solved only by infecting a perfectly 

 healthy plant with the spores of a fungus, and observing the 

 results. The external circumstances which favour and hinder 

 the development of the disease must also be noted. Such 

 observations are troublesome and difficult, and demand great 

 care and foresight to avoid deceptive conclusions. Much 

 information has, however, already been obtained, and every 

 year is adding to our knowledge of the subject. 



A glance must be given over the species and modes of attack 

 and the spread of a disease. In the case of diseases which 



