294 INTRODUCTION 
In organization the cells of Spermatophytes do not differ essen- 
tially from those of most Thallophytes. Excepting in the very 
lowest forms, the cellular structures of Thallophytes are similar 
to those of the Spermatophytes as a study of the unicellular 
Thallophyte in Figure 254 will show. This one-celled plant is 
composed of protoplasm, which is the living substance, and a 
wall, which encloses the protoplasm. The protoplasm, as in the 
cells of higher plants, consists of nucleus and cytoplasm. The 
Fig. 254.—A 
one-celled Thallo- 
phyte, Pleurococcus 
vulgaris. n, nucleus 
showing nuclear 
membrane, chro- 
matin, and a nucle- 
olus. c, cytoplasm, 
in which there is a 
large lobed chloro- 
plast. x 800. 
From Strasburger. 
nucleus, usually globular in shape, is en- 
closed by a nuclear membrane and contains 
one or more nuc!eoli (small globular bodies) 
and chromatin (the chunky or granular sub- 
stance scattered about in the nucleus). In 
addition to nucleoli and chromatin, the nu- 
cleus contains nuclear sap (water containing 
sugar, salts, and other substances in solu- 
tion). The cytoplasm, the protoplasm out- 
side of the nucleus, is vacuolate and has its 
outer border so modified as to form a mem- 
brane, which, unless the protoplasm is 
shrunken, is tightly pressed against the cell 
wall. Water and solutions enter the proto- 
plasm through this cell membrane by the 
processes of osmosis and diffusion. All of 
these cellular structures have practically the same function here as 
in the cells of the higher plants. In this particular unicellular plant 
there is a chloroplast, which, like the chloroplasts in the food- 
making cells of leaves, is a special protoplasmic body saturated 
with a green pigment (chlorophyll), which enables it by utilizing 
the sunlight to carry on photosynthesis, that is, to form sugar 
from carbon dioxide and water. 
Although consisting of a single cell, this plant performs most of 
the functions which the most highly organized plants perform, 
but in a simpler way. In absorbing water and mineral elements 
directly from its surroundings, it performs the function of roots. 
In carrying on photosynthesis, it performs the function of leaves. 
By dividing it gives rise to new individuals and thereby performs 
the function of reproduction, which is the function of flowers. In 
such a simple plant there is no function comparable to that of 
a stem, for there are no distant parts, such as leaves and roots, 
