THE FUNGI 383 
Yeast plants grow in dilute saccharine solutions containing 
dissolved nitrogenous substances such as beerwort, Pasteur’s 
solution, grape juice, etc. Here they are constantly wasting 
away and as constantly being built up. The question may well 
arise: ‘‘How do they obtain the material necessary for growth 
and repair?” The answer, in a general way, is not difficult. 
The fluid in which they live is a solution of sugars and of nitrog- 
Fic. 283.—Yeast, Saccharomyces cerevisiae, the variety known as brewers’ bot- 
tom yeast; a, spore formation; 6, elongated cells. (After Schneider, Pharmaceutical 
Bacteriology.) 
enous and other matters. The cell walls are readily permeable. 
Food substances diffuse through it into the cell, and by a series 
of changes (which, indeed, it is no easy matter to understand) 
are converted into new living substance. The waste products 
likewise diffuse readily outward. This method of nutrition is 
called saprophytic, and the yeast plant is said to be a saprophyte. 
A striking fact must be briefly mentioned in connection with 
the metabolism of yeast. Many organisms exercise a profound 
