CHAPTER XV 
SUMMARY AND CONCLUSION 
ALL plants agree closely in their essential cell struct- 
ure, the typical cell having a cellulose membrane and 
a single nucleus. This simple type of cell constitutes 
the whole plant in many low forms and makes up the 
young parts of the higher plants. From it are derived 
the variously modified cell forms constituting the spe- 
cialized tissues of these higher plants. In the lowest of 
all plants, the Bacteria and their blue-green allies the 
Schizophycez, the cell does not always show a cellulose 
membrane and the nucleus is imperfectly developed. 
The lowest plants are mainly aquatic, and it is ex- 
ceedingly probable that this is the primitive condition 
for plant life. Leaving aside the Schizophytes, whose 
affinities are somewhat doubtful, the peculiar group of 
motile green alge, the Volvocinez, probably represents 
more nearly than any existing forms the ancestral type 
of all the higher green plants. These ciliated alge 
are also probably related to certain colorless flagellate 
Infusoria, which in turn may represent the starting- 
point for the whole group of Metazoa among the ani- 
mals. It is not unlikely that the separation of the two 
great branches of organisms, plants and animals, took 
place among the Flagellata. 
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