With regard to the microscopic appearance of the film cells, these are, as a rule, very long shaped in the older growths, and the cells tend to assume irregular forms; now and then they also develop a mycelium. Spores are found in the film cells as an exception, namely, in that species of film formation called the yeast ring. The latter is formed, as mentioned above, along the edge of the liquid surface on the wall of the flask and is very marked in certain species. For instances of the varied appearance of the film cells in different species as well as when compared with bottom yeast, see figures 65 to 76. The difference in the film cells of these species is most noticeable at 13° to 15° C. The highest and lowest temperatures at which film formation has been observed by Hansen in the different species were given by him in 1886 in the above-mentioned treatise. They are as follows: Sacch. cerevisiæ I.: 33° to 34° C. and 6° to 7° C. Sacch. Pastorianus I., II. and III.: 26° to 28° C. and 3° to 5° C. Sacch. ellipsoideus I.: 33° to 34° C. and 6° to 7° C. Sacch. ellipsoideus II.: 36° to 38° C. and 3° to 5° C. As regards the time required for the development of the film, Sacch. ellipsoideus II. is especially noticeable, this species developing a very vigorous film in about ten days —often earlier at 22° to 23° C. The other five species require a much longer time. Will has shown that the different generations of cells formed in the film exhibit dissimilarities. His results, however, cannot be further referred to in a limited book like the present. We therefore refer those who desire to study these conditions more particularly to his papers (1895 and 1899) mentioned in the literature review for this section. With regard to the limiting temperatures of film formation for four of the species of beer bottom yeasts examined by him, a minimum temperature was FIG. 77.-Saccharomyces Ludwigii, Hansen. Mycelium and spore formation from very old cultures in cherry juice and yeast water respectively. I. to IV., Mycelia or fragments of such with broad thick septa. V., An irregular, branched small mycelium completely devoid of septa. VI., Mycelium threads, also with broad septa; in each cell (ascus) 4 spores. 1000. (After Hansen.) found at 4° to 7° C. for two, and at 7° to 10° C. for the other two, while he found the maximum temperature for three species to be near 30° C., and, for the fourth, 28° C. When a growth has formed a film, special chemical changes take place in the nutrient liquid. Thus wort usually assumes a light yellow colour under these conditions, and develops a disagreeable smell and taste. The formation of the film, however, is not always to blame for this, since beer which has stood for a long time, and on which no film has appeared, may likewise assume the above properties. According to Raymann and Kruis, the film cells, if allowed FIG. 78. Resting Cells Germinating. (After Will.) to remain for a long time, are capable of converting the alcohol formed in the culture liquid into carbonic acid and water by oxidation. Hansen showed that, under certain conditions, saccharomycetes can also develop a mycelium provided with septa. He observed such an one first in Sacch. Marxianus and Sacch. Ludwigii (Fig. 77), and afterwards in some other species. It occurs chiefly in old films and in old cultures on solid nutrient material. A mycelium of this kind then frequently shows forms similar to Dematium or Monilia. |