270 FOUNDATIONS OF BOTANY 
326. Multiplication of Yeast.—It is worth while to notice the 
fact that yeast is one of the few cryptogams which have for ages 
been largely cultivated for economic purposes. Very recently yeast 
producing has become a definite art, and the cakes of compressed 
yeast so commonly sold afford only one instance of the success 
that has been attained in this process. 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 off- 
shoots. The whole process ‘is called reproduction by budding. It is 
often possible to trace at a glance the history of a group of cells, 
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 (ascus). 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 
acquaintance 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 SLUDY~ OF FAVS 
327. Occurrence. — Physcia is one of the commonest lichens. It 
grows attached to the bark of various trees. 
328. The Thallus. — Physcia consists chiefly of an irregularly 
expanded growth somewhat leaf-like in texture. It is best to be wet 
for study. Is it separable from the bark to which it is attached or 
is it combined with it (incrusted) ? Describe the general outline of 
the margin, the general color, and any special variations of color 
above, also below, How is the thallus attached to the bark? 
