628 VITAL ACTIONS OF THE LOWER FUNGI. 



a parasitismus ; on the contrary, there are certain para- 

 sites which can live in a plant or an animal for a long 

 time without in any way influencing its normal exist- 

 ence, and indeed in many cases the parasitismus can form 

 an advantageous symbiosis for the host, as, for example, 

 in the lichens, which consist of a combination of algre 

 and fungi, in which the chlorophyl-bearing algae pro- 

 vide carbon compounds which can be utilised by the 

 fungus, while the latter takes up mineral nutrient 

 materials from the substratum, and thus supplies them 

 for the common nourishment. 



As a rule, however, the advent of parasites, and more 

 especially of the parasitic fungi, is accompanied by more 

 or less severe injury to the organisms which they attack, 

 A number of the most devastating diseases of plants, 

 animals, and man, have been proved with complete cer- 

 tainty to be due to parasitic fungi, and our chief motive 

 and real aim in studying the morphology and biology of 

 the lower fungi is the fact that they can excite disease. 

 Classification Among the large number of the fungi which are 

 parasitic capable of leading a parasitic life, we notice, when we 

 fungi. study them accurately, differences as regards the mode 



of the parasitismus which render it easier to understand 

 this peculiar phenomenon. Many of the parasitic fungi 

 are only able to lead this kind of existence, and these 

 can carry on their vital functions, either not at all, or 

 only in a very limited manner, on dead substrata ; other 

 fungi, on the contrary, also flourish as saprophytes, and 

 their parasitic existence is in fact only a continuous ex- 

 tension of the wide limits within which they can carry 

 on their life and action. 



De Bary has sharply defined these differences, and has 

 based on them the following classification of the parasitic 

 fungi : 



Obligatory ! Obligatory parasites. These require a parasitic 



parasites. jjf e j n or( j er to attain their full development. They may 

 be divided into two classes, namely, (a) Strictly obligatory 

 parasites, which only occur in nature as parasites, and 

 can be cultivated on dead substrata only under con- 

 ditions artificially produced in the laboratory ; and (b) 



