470 THE FUNGI WHICH CAUSE PLANT DISEASE 



In plant pathology and in systematic mycology it becomes 

 necessary to classify, for convenience of reference and designation, 

 these multitudinous conidial forms of which the higher spore 

 form is as yet unknown, which may exist now or which may have 

 existed only in the more or less remote past. From analogy it 

 is probable that most of them pertain to the Ascomycetes, though a 

 few may find place among other classes. 



This whole group of forms, which is characterized chiefly by the 

 imperfection of our knowledge of them, is classed together under 

 the name Fungi Imperfecti. 



The Fungi Imperfecti are in a temporary way divided into 

 orders, families, genera and species as are other fungi, with full 

 recognition of the fact that future research will result in many 

 cases in the disclosure of higher spore forms and the consequent 

 removal of species to their proper place in the general scheme of 

 classification. 



Recognizing the tentative nature of the genera in the Fimgi 

 Imperfecti these are spoken of as "form-genera." 



Pathologically, the Fungi Imperfecti are of high importance, often 

 occurring on leaves, stems, fruit, wood, bark, etc., as active parasites, 

 though very many are also saprophytes. Upon leaves they are 

 particularly common causing diseased areas known as "leaf spots." 



The Fun^ Imperfecti display three principal types of fructifi- 

 cation, pycnidia, acervuli and hyphae. 



Pycnidia are more or less spherical, hollow sporocarps on the 

 inside of which conidia are borne on stalks (conidiophores) arising 

 from the base or base and sides. Figs. 349, 354. The pycnidium 

 may be of various colors though it is most commonly black or dark; 

 it may be superficial or imbedded, and with or without a beak 

 (rostrum). The opening for the escape of the spores (ostiole) may 

 be narrow, or wanting or it may be very large, round, irregular, 

 etc. The walls vary from extremely delicate to very thick, smooth 

 or variously provided with hairs, spines, etc. 



As need arises, it is common to speak of micro-pycnidia, and 

 macro-pycnidia. Pycnidia with very small spores are sometimes 

 called spermogonia especially if the spores do not germinate, a cus- 

 tom to be deprecated. 



The acervulus may be regarded as a pycnidium without its wall. 



