300 PART III. THE CLASSIFICATION OF PLANTS. 



spore is spherical, or oval, or rarely filamentous (e.g. Claviceps, 

 Fig. 212). The wall generally consists of exosporium and endo- 

 sporium : the protoplasm generally contains oil -drops. 



The germinating ascospore usually gives rise directly to the game- 

 tophytic mycelium ; but in some forms (e.g. Polystigma) it gives 

 rise to a promycelium bearing small gonidia, termed sporidia, and it 

 is from the sporidium that the gametophytic mycelium is developed. 



The Ascomycetes may be classified as follows : 



Order I. Gymnoasceae : asci without any investment, or with only a rudi- 

 mentary investment, either solitary, or forming a hymenial layer. 



The typical members of this group are Erem.ascus (Fig. 206), Gymnoascus, 

 and Exoascus parasitic on various trees. 



It is now customary to place in this order the family of the SACCHAEOMYCETES, 

 or Yeast-Fungi, which is familiar on account of the alcoholic fermentation of 

 saccharine solutions which some of its members excite (e.g. Saccharomyces Cere- 

 visiae used in brewing, and S. ellipsoideus, which causes the fermentation of the 

 grape-juice in the manufacture of wine). The plant is usually a single small 

 spherical or oval nucleate cell, and multiplies rapidly by budding (Fig. 210). 

 When budding is proceeding very rapidly, the suc- 

 cessive cells may remain coherent for a time ; but a 

 true mycelium is only rarely found, as in S. Myco- 

 derma, which forms a scum on decomposing wine 

 and beer. 



Under certain conditions, particularly the absence 

 of a sufficient supply of food, the plant forms spores. 

 FIG. 210. Growing cells Usually four spores are formed in a cell, by free 

 of Yeast (Saccharomyces cell-formation, from a portion of the protoplasm, the 

 CeremstoO ; the clear spaces ^ remaini &s & ; etal j of epipla8m . The 

 in the cells are vacuoles. & * 



/ x 300 j spores surround themselves with a membrane, and 



are set free by the disorganisation of the wall of the 



cell. The spores retain their vitality under conditions, such as desiccation, 

 absence of food, extremes of temperature, etc., which would prove fatal to the 

 Yeast-plants. The spores germinate, on attaining appropriate conditions of 

 moisture and temperature, and give rise to Yeast- cells by budding. 



Inasmuch as the formation of the spores in a Yeast-cell takes place in the 

 same manner as the formation of spores in an ascus, the Yeast-cell may be re- 

 garded as an ascus. It is on this account that the Saccharomycetes are in- 

 cluded in the Ascomycetes, and in the Gymnoascese on account of their naked 

 asci. They are, however, reduced and sexually degenerate forms. 



It must be borne in mind that cells very similar to those of the true Saccha- 

 romycetes, multiplying in the same manner, and often capable of exciting the 

 alcoholic fermentation of sugar, may be formed by gemmation from the gonidia 

 of various kinds of higher Fungi (e.g. Mucor racemosus, Penicillium glaucum, 

 some Entomophthoraceae, Ustilagineae, and Basidiomycetes) under special con- 

 ditions. These Yeast-like cells, however, grow into mycelia under appropriate 

 treatment. However, it is still a question whether all the forms of Saccharomy- 

 cetes may not be merely secondary gonidial forms or gemmae of mycelial Fungi. 



