THE FUNGI: ASCOMYCETES AND BASIDIOMYCETES 275 



Saccharomycetales 



The organisms included in this order are of somewhat diverse character, 

 but they are all predominantly unicellular. They have probably originated 

 from more than one group of Fungi and are now classed together because of 

 their similarity of form. We know that some Fungi which are not at all closely 

 related to this order can adopt under certain conditions a yeast-like habit 

 of growth, so that the unicellular condition may have been arrived at along 

 more than one evolutionary line. It is at any rate reasonably certain that 

 this feature is secondary or derived, so far as this group is concerned and not 

 primitive in the evolutionary sense. 



Some members of the order possess a definite sexual process, but in many 

 it is wholly lacking. In the latter case reproduction is entirely vegetative. 

 It may be that such species are really heterothallic, in which case the chances 

 are very great against the two strains meeting to conjugate, except in artificial 

 culture. 



Saccharomyces cerevisiae (The Beer Yeast) 



The cells of Saccharomyces are very minute and either spherical or slightly 

 oval. They are about lOfx in diameter and are clothed with a delicate 

 membrane, which appears to be a compound of two polysaccharides, glycogen 

 and mannan, in combination with phosphoric acid. Cellulose is not present. 

 There is abundant cytoplasm containing several small vacuoles in which 

 are often grains of volutin (so called from its having been discovered in 

 the bacterium Spirillum volutans), which is ribo-nucleic acid or one of 

 its salts. 



During fermentation glycogen increases in the vacuoles so that they 

 stain dark brown with Iodine. This carbohydrate is the chief metabolic 

 product and reserve material of the yeast cell. 



The method of nuclear division is still uncertain, some authorities 

 maintaining that it is entirely amitotic, while others claim the existence of a 

 simple form of mitosis with two chromosomes. The latter view seems the 

 more probable in view of the life-cycle described below (Fig. 266). 



Species of yeast have been kept in cultivation by mankind since early 

 times, and are widely used in fermentation processes, e.g., in beers, wines 

 and milks, and for baking. Many species are, however, to be found wild on 

 the surfaces of higher plants, especially on the fruits. When the latter drop in 

 autumn the yeasts pass into the soil where they spend the winter. In spring 

 they are splashed by rain back on to the plants, thus completing the cycle. 

 We will deal with the chemistr\^ of the fermentation of sugars by yeasts 

 under Respiration in Plant Physiology (Volume III.) 



Asexual Reproduction 



Reproduction in Saccharomyces takes place in two ways. The first is by 

 the formation of buds on the vegetative cells, and the second is by means of 



