266 HEREDITY IN SACCHAROMYCETES. 



increased at the expense of the bottom-cell form until the latter 

 was entirely suppressed. 



In contrast with the asporogenic varieties which, as we have 

 already seen, are modifications produced by the influence of a 

 known external factor, high temperature, the appearance of top- 

 yeast cells in a typical bottom yeast must be relegated to the 

 category of variations to which the name mutation was given by 

 de Vries, and comprising all sudden variations due to unknown 

 causes. In most cases the properties of these mutation varieties 

 are hereditary, as we have already learnt in the case of the tem- 

 porary variations. Thus, the variations in the cell form, the 

 shape and size of the spores, and also the production of mycelium 

 observed by Lepeschkin, must be classed as mutations. The great 

 difference existing between a transformation and a mutation is 

 that the former is produced gradually, the latter suddenly. Both 

 may form the starting-point of new species or races. Throughout 

 the entire vegetable kingdom only very few instances are known 

 where a new variety, capable of transmitting its newly acquired 

 properties permanently to its offspring, has resulted from a 

 transformation brought about by external influences; in fact 

 Hansen's researches on asporogenic varieties constitute the sole 

 experiment performed in this connection. 



276. Practical Results of the Researches on Variation. 

 Occurrence in Brewing- Practice. 



Before bringing this chapter to a close we will examine the 

 practical bearing that the results of the foregoing investigations 

 have on brewing, and also consider the occurrence of variations of 

 yeast type in practice. 



By employing an asporogenic variety of pitching yeast in the 

 brewery, the detection of wild yeasts by spore analysis is simplified. 

 It will be remembered (pp. 135, 136, vol. ii.) that this method of 

 examination is based on the fact that, at a certain temperature, 

 sporulation occurs sooner in wild yeasts than in culture yeasts. 

 When, however, an asporogenic yeast is used for pitching, the 

 mere presence of asporogenic Saccharomyces cells of any kind will 

 suffice to reveal an extraneous yeast. The fact that an asporogenic 

 yeast will produce just as good beer as the original form has been 

 demonstrated by Hansen, who obtained a good normal beer with 

 an asporogenic variety obtained from Carlsberg bottom yeast 

 No. 2 by the treatment already described. It must not, however, 

 be forgotten that in many instances the behaviour of the asporo- 

 genic variety in practice will differ appreciably from that of the 

 original yeast from which it was produced 



WILL (XXVIII.) has reported an abnormal fermentation 

 phenomena resulting from the presence of film cells or their 

 descendants in the pitching yeast; and A. JORGENSEN (II.) has 



