246 ESSENTIALS OF BOTANY 
becomes still lighter as the gas expands in the oven. Cakes 
of compressed yeast are nowadays generally used: in bread 
making. It is important that they should be as fresh as 
possible, since when they are old they are likely to contain 
some wild yeasts and many bacteria. These are pretty sure 
to injure the quality of the bread made from such yeast. 
RHIZOPUS, ONE OF THE ALGAL FUNGI 
317. Occurrence. — The common bread-mold, Rhizopus 
nigricans, is found upon bread or cake left in a warm 
damp place, or on sweet potatoes, squashes, or bananas. 
Its spores are likely to be present in the dust of houses, 
and many lie dormant for months, producing when they 
alight on favorable food material a luxuriant crop of the 
downy coating of white silky threads so familiar to all. 
818. Gross Structure. — Place some bread} in a soup plate, wet 
thoroughly, cover with a bell-glass, and leave for some days in a dark 
warm place, renewing the water if necessary. When a considerable 
growth of mold has appeared, examine with the magnifying glass an 
undisturbed portion as it grows from the surface of the bread. Note: 
(a) The tangled network, or mycelium, visible on the surface of 
the bread (and really extending a good way down into it). 
(b) The size and appearance of the separate threads, or hypha, of 
which the mycelium is composed. 
(c) The presence of certain erect aérial hyphe bearing minute 
globular spore-cases at their tips. Describe the color of the youngest 
spore-cases and that of the largest and oldest ones. 
319. Minute Structure. — Wet a little of the mold with alcohol (to 
remove the air from it) and then mount in water and examine with 
m.p. Note: i 
(a) The mode of growth of the hyphe, whether branched or 
unbranched. 
1 Gluten bread is particularly adapted for this culture. 
