FUNDAMENTAL RESEARCHES. 255 



especially so far as Sacch. apiculatus is concerned, this species 

 being reproduced under these conditions less extensively than the 

 true Saccharomycetes. In addition, the latter will also stand a 

 longer immersion in water than Sacch. apiculatus. 



Saccharomycetes are found not only on the surface of the'ground, 

 but also in the thin layers of soil found above ground on trees, 

 brickwork, stones, &c., where they are protected from drought by 

 a stratum of moss, lichens and algre. These plants avidly absorb 

 water, and the under layer which, especially in the case of moss, 

 consists of dead residual matter readily cedes nutrient substances 

 to the water. In forests of deciduous trees the foliage affords 

 additional protection against drought ; but in the open fields 

 different conditions obtain, the Saccharomycetes occupying secon- 

 dary breeding-places there, being exposed to more or less extensive 

 desiccation. 



These studies on secondary breeding-places and on the behaviour 

 of the various species towards drought, furnished Hansen with an 

 explanation of the irregularities referred to above. When Sac- 

 cliaromycetes cannot be found in the primary breeding-places 

 (sweet, juicy fruit), sun, wind and weather have made their, 

 influence felt ; and when, on the other hand, they are found abun- 

 dantly in the open fields, the reason is that they have discovered 

 unusually favourable secondary breeding-places there, and have at 

 the same time been protected from desiccation. The secondary 

 breeding-places are of considerable importance by reason of their 

 extensive distribution. An examination of the behaviour of the 

 species towards temperature also explains in some respects what is 

 taking place in nature, Hansen having found that several species 

 are capable of reproduction when the surrounding temperature is 

 at freezing-point though under these conditions several months 

 are necessary for the production of a single generation, even when 

 the cells are situated in a favourable nutrient liquid. As a rule 

 reproduction ceases when the temperature falls to i-2 C., and 

 a much higher degree is necessary to enable it to proceed with 

 vigour. For this reason the number of yeast cells found in a 

 given spot varies according to the time of year, being greatest in 

 the fruit season, at which season the most favourable conditions 

 are found in respect of temperature, food-supply and moisture. 

 Among the primary breeding-places the soil furnishes its maximum 

 yield in autumn ; and it follows from what has been stated above 

 that the secondary breeding-places are active at the same time. 

 Afterwards, the fluctuations in the course of the year are con- 

 siderable, more especially, as already mentioned, where extensive 

 drying takes place. 



The principal result therefore is that the soil is the chief habitat 



of the Saccahromycetes in general (as it is in the case of Sacch. 



apiculatus} all the year round. From this starting-point the cells 



are transported by the aid of wind, rain, insects and other small 



VOL. ii : PT. 2 R 



