UNICELLULAR PLANTS, 9 
and unstable. 
The nucleus plays a prominent part in the life- 
history of the cell, and seems to be essential to its perfect devel- 
opment and greatest physiological efficiency. 
UNICELLULAR PLANTS. 
Yuast (Torula, Saccharomyces Cerevisic), 
The essential part of the common substance, yeast, may be 
studied to advantage, as it affords a simple type of a vast group 
of organisms of profound 
interest to the student of 
physiology and medicine. 
To state, first, the main 
facts as ascertained by 
observation and experi- 
ment : 
Morphological, — The 
particles of which yeast 
is composed are cells of a 
circular or oval form, of 
an average diameter of 
about x45, of an inch. 
Each individual torula 
cell consists of a trans- 
parent homogeneous cov- 
ering (cellulose) and gran- 
ular semifluid contents 
(protoplasm). Within the 
latter there may be a 
space (vacuole) filled with 
more fluid contents. 
The various cells pro- 
duced by budding may 
remain united like strings 
of beads. Collections of 
masses composed of four 
or more subdivisions (as- 
cospores), which finally 
separate by rupture of 
Fi. 2.—Various stages in the development of brewer's 
yeast, seen, with the exception of the first _in the 
series, with an ordinary high power (Zeiss, D. 4) of 
the microscope. The first is greatly magnified 
(Gundlach’s ¥; immersion lens). The second series 
of four represents stages in the division of a single 
cell; and the third series a branching colony. 
Everywhere the light areas indicate vacuoles. 
Fie. 3.—The endogonidia (ascospore) phase of repro- 
duction—i. e., endogenous division. 
6¢* 
Fic. 4.—-Further development of the forms represented 
in Fig. 3. 
the original cell wall, having thus become themselves inde- 
pendent cells, may be seen more rarely (endogenous division). 
