200 INTRODUCTION TO THE STUDY OF FUNGI 



the sporidia are septate in both directions. In all cases 

 these Fungi are destructive pests, by choking the pores or 

 stomata, although only nourishing on the external surface. 

 Such are the rose mildew, Sphaerotheca pannosa ; the hop 

 mildew, Sphaerotheca castagnei ; the pea mildew, Erysiphe 

 Martii, and others. The number of known species is about 100. 

 The next subfamily, Perisporieae, is more numerous, and 

 whereas the majority of the Erysipheae are confined to cool and 

 temperate regions, the majority of the Perisporieae are sub- 

 tropical. The species are not parasitical, or to a limited extent, 

 and the subiculum, when present, may be either colourless or 

 coloured, mostly the latter. The perithecia, although most 

 commonly subglobose, are sometimes depressed and lens-shaped, 

 and, except in Meliola and one or two smaller genera, without 

 appendages. The sporidia are more variable than in the first 

 subfamily, and hence fall under five sections : Hyalosporae, with 

 continuous hyaline sporidia ; Phaeosporae, with continuous 

 coloured sporidia ; Didymosporae, with uniseptate sporidia ; 

 Phraymosporae,'with. multiseptate sporidia; and Dictyosporae, with 

 muriform sporidia. Under these various sections the twenty- 

 five genera are distributed. This arrangement foreshadows the 

 principles upon which the several groups of Pyrenomycetes are 

 grouped ; that is to say, primarily on the basis of the sporidia, 

 whether coloured or hyaline, and whether continuous or septate. 

 It is unnecessary to occupy space by a synopsis or comparison 

 of the component genera, but simply to make reference to two 

 or three of the most important. Asterina, in its broadest 

 sense, is characterised by a flattened or lens-shaped perithecium, 

 seated on a dark radiating subiculum, with sporidia continuous 

 or septate, hyaline or coloured ; hence the principle adopted 

 almost universally by Saccardo, in other genera, of making 

 spore-characters of generic value is set aside, and only employed 

 for the distinction of subgenera. We have always contended 

 in favour of the Friesian system of adopting external characters 

 in the definition of genera, reserving carpological features for 

 subsidiary sections ; and hence the genus Asterina, as it finds a 

 place in Saccardo's Sylloge, will serve as an illustration of our 

 method, but not of that of Saccardo, to which it does not 

 conform, and would only do so by elevating the separate 



