398 



BOTANY 



PART II 



-a* 



Order 5. Exoasci (**) 



The most important genus of this group of Ascomycetes is Taphrina (including 

 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 recurrence of the disease takes place. The presence of the 

 mycelium in the tissues of the infected part causes the abnormally profuse 

 development of branches known as WnriiKs'- BROOMS. Taphrina Carpini 

 produces the abnormal growths occurring on the Hornbeam ; Taphrina epiphylla, 



those of .Units incana. Taphrina deformans 

 attacks the leaves of the Peach and causes them 

 to curl. Tii'/i/iriii-ii, 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, which occurs 

 without the formation of sexual organs, in- 

 dividual cells of the copiously-branched mycelium 

 ramifying between the epidermis and cuticle 

 of the infected part become greatly swollen. 

 These 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. 340). The numerous 

 ,,a I /(. Transverse asc i are closely crowded together. In conse- 

 section through the epidermis of an ce Q{ . their increaaed t resulting from 



infected plum, b our ripe asci, n } . <*., ' 



with einht spores, :! , 4 with yeast" an excessive absorption oi water, the asci become 

 like (-.onidia abstracted from the ruptured at their free extremities and eject the 

 spores ; st, stalk-cells of the asci ; spores. 



m, filaments of the mycelium cut ^ whi(jh bu( j iu watw Qr g 



transversely : cut, cuticle; ep, epi- . L - . ... ... 



lemiis. (After SADEBECK, x 000.) wlution, frequently germinate while still en- 

 closed within the asci (Fig. 340, a 3 , a 4 ), and 

 give rise by budding to yeast-like conidia, e.g. in Taphrina Pruni. 



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

 sexual orj^iius have become completely suppressed. 



UOfc 



. cut 

 .en 



Order 6. Saccharomycetes ^ Yeast-Fungi) ( 67 ) 



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

 simple unicellular Fungi which assume the form of spherical, oval, or cylindrical 

 conidia containing a single nucleus. They increase in number by budding 

 (Fig. 341). 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 conidia but contain a few spores. In .some yeasts a conjugation of 

 two cells accompanied by a nuclear fusion has been observed ( 68 ). In Saccharo- 

 myces Ludwigii the four spores in the ascus germinate and fuse in pairs l>y means of 

 a narrow conjugation- tube ; the latter elongates into a germ-tube from which yeast- 



