230 ELEMENTS OF BOTANY. 
water, and other mineral substances, as ordinary green plants can. It 
gives off no oxygen, but only carbonic acid gas, and is therefore to be 
classed with the saprophytes, like the Indian pipe among flowering 
plants, § 151. 
283. Multiplication of Yeast. — While yeast cells are under favorable 
conditions for growth, they multiply with very great rapidity. Little 
protrusions are formed at some portion of the cell wall, as the thumb of 
a mitten might be formed by a gradual outgrowth from the main portion. 
Soon a partition of cellulose is constructed, which shuts off the newly 
formed outgrowth, making it into a separate cell, and this in turn may 
give rise to others, while meantime the original cell may have thrown 
out other offshoots. The whole process is called reproduction by budding. 
It is often possible to trace at a glance the history of a group of cells, 
like those of the right-hand cluster in Fig. 198, II, the oldest and largest 
cell being somewhere near the middle of the group and the youngest and 
smallest members being situated around the outside. Less frequently 
the mode of reproduction is‘by means of spores, new cells (usually four 
in number), formed inside one of the older cells. At length the old cell 
wall bursts and the spores are set free, to begin an independent existence 
of their own. 
In examining the yeast cell, the student has been making the acquaint- 
ance of plant life reduced almost to its lowest terms. The very simplest 
plants consist, like the slime-moulds, of a speck of jelly-like protoplasm. 
Yeast is more complex, from the fact that its protoplasm is surrounded 
by an envelope of cellulose, the cell wall. 
THE STUDY OF BLACK MOULD.! 
284. Occurrence.— This mould may be found in abundance on 
decaying fruits, such as tomatoes, apples, peaches, grapes, and cherries, 
or on decaying sweet potatoes or squashes. For class study it may most 
conveniently be obtained by putting pieces of wet bread on plates for a 
few days under bell jars and leaving in a warm place until patches of the 
mould begin to appear.? 
1 Rhizopus nigricans. If any difficulty is experienced in procuring material for 
study, the common sage-green mould, Penicillium glaucum, can always be procured 
and propagated as described in Huxley and Martin’s Biology. 
2 Tt will always be found much easier to obtain a good crop of the desired mould 
by sowing its spores upon the wet bread that is used. Spores may be kept in- 
definitely, in a dry condition, for this purpose. Exposing the bread to a confined 
portion of the atmosphere of any place, e.g., a cellar, where the desired mould has 
previously flourished will insure a prompt growth of the mould anew. 
