422 PROTECTION AGAINST PLANTS. 



trees hitherto ascertained to be due to bacteria occurs in the 

 case of Pinus halepensis, Mill. 



By their presence in the soil, bacteria greatly assist vegeta- 

 tion by decomposing and dissolving organic refuse, and the 

 fact that a forest soil when exposed for some time to the action 

 of the sun's rays becomes less fertile than when it is con- 

 stantly sheltered by trees is in complete accordance with 

 Ward's researches. 



As already stated, bacteria, and certain fungi as well, also 

 protect forests by killing* insect pests in enormous numbers. 

 The only thallophytes which cause serious injury to plants 

 are fungi that are devoid of chlorophyll and therefore obtain 

 their nutriment from other dead or living organisms. In 

 the former case, they are termed saprophytes and in the 

 latter, parasites. Another class of fungi obtaining nourish- 

 ment from humus, but becoming attached to and rendering 

 abnormal the roots of many trees and shrubs, deserves notice. 

 Fungi growing on the exterior of other plants may be termed 

 epiphytic. 



The number of known fungi is very great, over 5,000 ; here 

 only those which affect the growth of forest plants will be 

 considered. 



2. Classification and Importance of Fungi from a Forest 



Point of View, 

 a. Saprophytic Fungi. 



As saprophytic fungi live on dead or dying organisms, they 

 do not cause disease, but follow or accompany an already 

 diseased condition of their hosts. Fortunately, the majority 

 of known fungi belong to this class. Fungi that are sapro- 

 phytic in certain cases, as Armillarea mellea, Vahl., on rotten 

 stumps of beech and other broadleaved trees, may be parasitic 

 on other trees, such as conifers. 



b. Parasitic Fungi. 



Parasitic fungi attack healthy plants, and either cause a 

 sickly condition or actual death to their hosts. Some parasitic 

 fungi subsequently become saprophytic in tissues which they 

 have killed. 



