56 EVOLUTION OF PLANTS 
third order, the Characew, is made up of very peculiar 
plants of doubtful affinities. 
THE SIPHONES 
This order contains a good many types differing a 
good deal among themselves and showing in some cases 
a high degree of special- 
ization. They differ 
from the other green 
forms in the almost 
complete absence of 
division walls within 
the plant body, although 
they can hardly with 
propriety be considered 
Bs i as strictly unicellular 
Fic. 11.—Part of a plant of Caulerpa since the protoplasm 
plumaris, one of the Siphonez, show- * 
ing external differentiation into stem, CONtalns a large number 
eth som mepoine s aodey of nuclei. The plant 
may be a simple tubular 
filament, or it may be extensively branched and forma 
body of considerable size showing a remarkable degree 
of external differentiation, actually mimicking the struct- 
ure of the higher plants in the development of stem, 
leaf, and root (Fig. 11); but even in such cases the 
hollow cavity of the thallus is undivided by partition 
walls. The wall is lined with a protoplasmic layer in 
which are imbedded the numerous nuclei and chloro- 
plasts. The division of the nuclei, of course, is not 
accompanied, as in most cells, by the formation of a 
division wall. 
