12 GENERA OF FUNGI 



when black and carbonous, and biatorine when bright-colored. The cellular 

 structure of the exciple, whether parenchymic or prosenchymic, marks the 

 distinction between Mollisiaceae and Helotiaceae, as well as between certain 



genera elsewhere. 



Both Hoehnel and Petrak have made use of minor differences in the 

 structure of the wall of perithecium and pycnidium, particularly the number 

 of layers and the character of the cellular pattern, but in the main these 

 must' wait much more extensive and systematic study before they can be 

 adopted. 



Ascus 



The criteria derived from the asci are primarily origin, number, oper- 

 culum, number of spores, and reaction to iodin. The method of origin is 

 essentially a family character, as exemplified in the fastigiate or corymbose 

 arrangement in Eurotiaccae as compared with the umbelloid in other 

 Pyrenomycetes, or in the inverted position typical of Trichothyriaceac. 

 The presence of a single ascus in the perithecium is of generic value, but it 

 occurs very rarely, except in the lower families, especially the Erysiphaceac. 

 The so-called monascous hymenium of M icro thy rial es is a wholly different 

 matter and like the arrangement in Myriangiaceae a consequence of other 

 changes. The presence of an operculum or lid is characteristic of the 

 Pczicaccac, Ascoholaceac and Helvcllaccae by contrast with the other fami- 

 lies of the Discomycctcs, but it is not here employed as a basis for ordinal 

 separation, since it is considered to make an unnatural division of the 

 phylum. 



The number of spores is a criterion long established by usage, though 

 it must be employed with some discretion in the case of lichens particularly, 

 where the variation in general is somewhat greater. It rests primarily upon 

 the overwhelming preponderance of the number 8; 4's and 16's are some- 

 times associated with 8 and in consequence are less dependable than larger 

 or smaller numbers. As a result, the numbers here regarded as warranting 

 segregation are 1-2, 8, 16-32, and the very high numbers designated as 

 myriosporous. Among the lichens, the low numbers sometimes vary within 

 a species or between closely related ones and hence lack validity. 



The question of the value of the color-test with iodin is still an open 

 one, but it has been employed with so much consistency and convenience by 

 Rehm, that it is continued here, pending more exact information as to its 

 validity. 



Paraphysis 



In the present instance, an endeavor has been made to definitize the use 

 of the term paraphysis by restricting it to the Ascomycetes and Pucciniales, 

 and employing pseudoparaphysis for more or less similar structures among 

 the Phomales and elsewhere. By contrast with these, other incidental fili- 

 form features, such as apophysis, periphysis and dendrophysis, etc., are 

 considered to have no particular diagnostic importance for genera at present. 



