THE FUNGI WHICH CAUSE PLANT DISEASE 



399 



sporophore is shown in Figs. 285, 286. Families are delimited 

 by the character of the sporophore, distribution of the hymenial 

 surfaces, presence of cystidia, size 

 and color of spores, and other more 

 minor points. 



In germination the spore pro- 

 duces a germ tube which develops 

 directly into a mycelium. In many 

 species the young mycelium is 

 conidia-bearing. 



Cytologically the group conforms 

 to the general description given on 

 pages 393, 394. 



The Agaricales are chiefly of in- 

 terest to pathologists as wood fungi 

 though in a comparatively few in- 

 stances they are found on herbs. 

 Upon wood they may do harm. 

 First, as root parasites, in which 

 case death may follow through in- 

 terference with absorption or an- 

 chorage. Second, as causes of 

 heart rots leading to weakness and 

 eventual overthrow of the tree. 

 Third, as parasites of sap wood, 

 cambium or bark leading to death 

 of a part of the host and often its 

 complete loss. 



In many instances the fungus 

 draws its subsistence from host 

 cells not actually alive and hence strictly speaking they are 

 saprophytes. Nevertheless, since their ultimate effect upon the 

 tree is to cause disease or death, from the practical viewpoint 

 these fungi are pathogenic. Many species, moreover, can start 

 their career on a host plant as saprophytes and after attaining a 

 stage of vigorous vegetative growth become truly parasitic. In 

 most instances they are woimd parasites, which cannot gain ac- 

 cess to the inner portions of the host through uninjured tissue 



Fig. 286. — Cross section of the gill 

 showing basidia, sterigmata and 

 spores, also a cystidium stretch- 

 ing from one ^ to the next. 

 After Buller. 



