204 COMPARATIVE MORPHOLOGY OF FUNGI 



repeated drying and wetting) tears the hyphae with which it was attached 

 to the substrate. Apparently, in these two genera, the appendages 

 serve for passive dissemination; possibly they also favor clinging to a new 

 substrate, as in some species of Uncinula they are also sticky. 



The cooperation of the appendages in the liberation of the perithecia 

 has only been demonstrated in Phyllactinia. The sac at the base of the 

 appendage is thick walled on the upper side and thin walled on the lower 

 side (Fig. 129, 1). In drought, the lower side wrinkles and bends down 

 like a joint, thereby the perithecium is raised from the surrounding 

 mycelium as upon stilts (Fig. 129, 2). Experiments in a desiccator have 

 shown that the force developed is comparatively large: Four perithecia 

 easily raise a cover glass of medium thickness. The clinging of the 

 perithecia to the new substrate is not accomplished in Phyllactinia by the 

 appendages but by peculiar tufts of hyphae at the top of the perithecia 

 which swell to a hygroscopic, foamy drop of slime (Fig. 129, 2) and 

 apparently, upon drying, attach the perithecia firmly. 



A review of the more important genera here discussed (taken in part 

 from Arnaud, 1921) is presented in the following scheme: 



Sphaerotheca Podosphaera 

 | Microsphaera 



Phyllactinia Erysiphe Uncinula 



Aspergillaceae 

 Diagram XXI. 



The polyascous Leveillida-Erysiphe group is regarded as primitive 

 and the monascous Sphaerotheca-Podosphaera group, as derived. With 

 this development, a decrease of intramatrical and increase of extra- 

 matrical mycelium occurs, i.e., a transition from endoparasitism to ecto- 

 parasitism, and a consequent increase in the ability of the conidiophores 

 to form spores. The Erysiphe- Sphaerotheca series is distinguished by 

 undifferentiated, the Uncinula-Podosphaera series by characteristic 

 appendages. 



Several representatives of these genera cause plant disease, as Erysiphe 

 graminis on cereals, Uncinula necator (Oidium Tuckeri) on grapes, 

 Sphaerotheca mors-uvae on gooseberries, S. pannosa on roses, S. Humuli 

 on hops, cucumbers, etc., Podosphaera leucotricha on apples and Micro- 

 sphaera alphitoides (Oidium quercinum) on oaks. 



Perisporiaceae. — In contrast to the Erysiphaceae, this family lacks 

 sharp limits. It begins with forms with Perisporial characters, develops 

 toward the Sphaeriales type and merges into this order in the Amphi- 

 sphaericaceae and its relatives. This will be briefly discussed in four 

 genera, Lanomyces, Balladyna, Meliola and Parodiopsis. 



