134 



MYCOLOGY 



and the diseases which they cause will be described later. Exoascus 

 pruni (Fig. 42) is the cause of an important disease of plum trees, 

 producing the so-called plum pockets. It also attacks Pruniis domestica 

 and P. padus in middle Europe, and P. domestica and P. virginiana in 

 North America. Exoascus communis attacks the fruits of several 

 American species of Prunus among them P. maritima. Exoascus 

 alnitorquus infests the pistillate spikes and cones of species of alder 

 (Alnus), such as Alnus glutinosa and A. incana in middle Europe, and 

 A. incana and A. rubra in North America, causing an enlargement of 

 the fruit scales into twisted, tongue-like, reddish outgrowths. Exoascus 

 deformans is the cause of peach-leaf curl. Exoascus cerasi is responsible 

 for the formation of witches' brooms on the cherry. The genus 

 Taphrina causes witches' brooms and leaf spots. Taphrina purpuras- 

 cens attacks the leaves of a North American sumac, Rhus copallina, 

 causing a puckering of the leaves with the formation of a reddish- 

 purple color. T. aurea (Fig. 42) forms yellow blotches on the leaves of 

 several European and North American poplars, viz., Populus nigra and 

 P. italica of Europe, and P. Fremontii, P. grandidentata and P. deltoides 

 of North America. T. Laurencia causes witches' brooms on a fern in 

 Ceylon, Pteris quadriaurita. 



Suborder B. Saccharomycetiineae. — A true filamentous mycelium 

 is absent in the fungi of this suborder. The plants are single-celled 

 and reproduce by budding, or gemmation. Occasionally under ex- 

 perimental treatment where the culture media are varied, the cells 

 develop into hyphae and together form a myceHoid growth. Spore 

 formation consists in a single cell, developing one to eight spores. 

 It, therefore, may be looked upon as an ascus and the spores are as- 

 cospores. Many of them cause fermentation. 



Family i. Saccharomycetace^. — Many species of the genus 

 Saccharomyces are called generically yeasts, and are of economic 

 importance, because they induce the alcoholic fermentation of car- 

 bohydrate substances. The action is accompl shed through a soluble 

 enzyme formed in the protoplasm of the yeast cell, and first isolated 

 by Buchner by grinding the yeast cells in sand and extracting the 

 ferment zymase. The general shape of yeast cells is oval, ellipsoidal, 

 and pyriform (Figs. 43, 44). The cell wall is well defined and consists 

 of modified forms of cellulose which may be called fungous cellulose, 

 because it does not react to the reagents used for true cellulose. This 



