THE PIN-MOLD FUNGI 495 
alge. The fission fungi, therefore, are regarded as descend- 
ants of the fission-alge (as the blue alge are sometimes called) 
which have adopted a saprophytic or parasitic mode of life. 
All plants which contain chlorophyll and, like the alge, 
make all their own food by means of sunlight, are termed 
holophytes;: while those which feed upon organic materials 
either as saprophytes or parasites are distinguished as hystero- 
phytes.2, Doubtless in consequence of their change of habit 
these hysterophytic fission-plants have not only altered their 
relation to sunlight, but have become more or less reduced 
in size. Certain species of the group are, so far as known, 
the smallest of living things. Multiplication solely by fission 
characterizes the class. 
179. The yeast fungi (Class Saccharomycetes). Alco- ~ 
holic fermentation, or the conversion of a carbohydrate into 
alcohol and carbonic acid gas, such as takes place in the 
manufacture of beer and wine and in the raising of bread, 
is usually accomplished by means of yeast. This consists 
of unicellular fungi (Fig. 151, a-d). The usual method of 
reproduction differs from fission in that new cells arise as 
small protuberances or buds which eventually attain the 
size of the parent cell. Several resting spores are formed 
in a single cell (e, f) and these germinate by budding (g, A). 
There is reason to believe that yeast-plants represent merely 
a stage in the life-history of more highly developed fungi, 
which, however, have the power of perpetuating themselves 
indefinitely in the simple ways described, much as we have 
seen to be the case with wall-stain alga. Whatever may prove 
to be their true relationship to other fungi the species of 
yeast are conveniently placed provisionally in a class by 
themselves composed of unicellular forms, which reproduce 
only by budding and the formation of spores by internal cell- 
dwision. 
180. The pin-mold fungi (Class Zygomycetes). Various 
fermentations or putrefactions affecting bread, preserves 
or other food, are often due to so-called ‘‘pin-molds” like 
the Mucor shown in Figs. 324, 325, 326. A spore falling 
1 Hol’o-phyte < Gr. holos, whole; phyton, plant. 
2 Hys’ter-o-phyte < Gr. hysteros, coming after. 
