DIV. 



THALLOPHYTA 



449 



T 



Order 6. Exoasceae ( r2 ) 



The most important genus of this group of Ascomycetes is Taphrina (includ- 

 ing Exoascus), the species of which are parasitic on various trees. They develop, 

 in part annually, beneath the cuticle of the 

 leaves, causing discolorations of these organs ; 

 their mycelium persists during the winter in 

 the tissue of the host, so that a constant re- 

 currence of the disease takes place. The presence 

 of the mycelium in the tissues of the infected 

 part causes the abnormally profuse develop- 

 ment of branches known as WITCHES'-BROOMS. 

 Taphrina Carpini produces the abnormal growths 

 occurring on the ^Hornbeam ; Taphrina Cerasi 

 those on Cherry trees. Taphrina deformans 

 attacks the leaves of the Peach and causes them 

 to curl. Taphrina Pruni is parasitic in the 

 young ovaries of many species of Prunus, and 

 produces the malformation of the fruit known 

 as "Bladder Plums," containing a cavity, the 

 so-called "pocket," in the place of the stone; 

 the mycelium persists through the winter in 

 the branches. In the formation of asci, the 

 copiously-branched mycelium ramifies between 

 the epidermis and cuticle of the infected part. 

 The individual cells of the mycelium become 

 greatly swollen and grow into club-shaped tubes, 

 which burst through the cuticle and, after cutting 

 off a basal stalk-cell, are usually converted into 

 asci with eight spores (Fig. 394). As in other Ascomycetes the young ascus has 

 two nuclei which fuse, and the resulting nucleus undergoes three divisions to give 

 the nuclei of the eight spores. The numerous asci are closely crowded together. 



The spores, which bud in water or sugar solution, frequently germinate whil 

 still enclosed within the asci (Fig. 394 3 , 4 ), and give rise by budding to yeast- 

 like conidia, e.g. in Taphrina Pruni. 



The Exoasceae are perhaps to be regarded as reduced Ascomycetes, in which the 

 sexual organs have become completely suppressed. 



FIG. 394. Taphrina Pruni. Transverse 

 section through the epidermis of an 

 infected plum. Four ripe asci, o lt ao 

 with eight spores, a 3 , 04 with yeast - 

 like conidia abstricted from the 

 spores ; gt, stalk-c^lls of the asci ; m, 

 filaments of the mycelium cut 

 transversely ; cvt, cuticle ; ep, epi- 

 dermis, (x 600. After SADEBECK.) 



Order 7. Saccharomycetes (Yeast Fungi) ( 73 ) 



The beer, alcohol, and wine yeasts included in the genus Saccharomyces are simple 

 unicellular Fungi which have the form of spherical, oval, or cylindrical cells contain- 

 ing a single nucleus. They increase in number by budding (Fig. 395). No mycelium 

 is formed, though sometimes the cells remain for a time united in chains. With free 

 access of oxygen and at a suitable temperature yeasts form asci when the nutrient 

 substratum is exhausted ; the asci externally resemble the yeast-cells, but contain a 

 few spores. In some yeasts a conjugation of two cells accompanied by a nuclear 

 fusion has been observed. In Saccharomyces Ludwigii the four spores in the ascus 

 germinate and fuse in pairs by means of a narrow conjugation-tube ; the latter 

 elongates into a germ-tube from which yeast-cells are abstricted. In the ginger- 

 beer yeast (Zygosaccharomyces} and in Schizosaccharomyces the yeast-cells conjugate 



2G 



