4 EVOLUTION OF PLANTS 
ymals the so-called “Monera,” which were formerly 
supposed to be composed of structureless protoplasm, 
are now known to possess a nucleus, so that we cannot 
assert positively that any known forms consist of un- 
differentiated protoplasm as was once supposed to be 
Fic. 1.— A, a cell from a hair at the 
pase of the stamen of a spider- 
wort (Tradescantia), showing 
the parts of a typical plant cell ; 
w, the cell-wall; pr, the proto- 
plasm in which is imbedded the 
nucleus, n, with the nucleolus, 
nu; v, vacuoles, spaces filled 
with watery cell-sap. B, a Des- 
mid (Cosmarium), a plant con- 
sisting of a single cell; pl, one 
of the chloroplastids; n, the 
nucleus. 
the case. As a rule the 
protoplasm is segregated in 
masses of definite form, usu- 
ally furnished with a more 
or less evident envelope and 
provided with a special 
structure, the nucleus. 
These nucleated masses of 
protoplasm are generally 
called cells, although the 
name is occasionally re- 
served for such as are pro- 
vided with a single nucleus 
and a definite membrane, 
each nucleus with its ac- 
companying protoplasm be- 
ing designated an “energid.” 
We shall, however, for con- 
venience’ sake use the term 
cell in its ordinarily accepted 
sense. 
In all but the lowest forms of life the cells always show 
at least two parts, the cell-plasm or cytoplasm, and the 
nucleus. 
The latter is usually of definite form, globular 
or lenticular (Fig. 1, A, x), and bounded by a definite 
membrane, which, however, is apparently not chemically 
different from the cytoplasm in which it is imbedded. 
