20 EVOLUTION OF PLANTS 
the cell increases in bulk, and sooner or later divides 
to form new ones which become at once new individuals, 
so that the single cell exhibits all the characteristics of 
a typical plant; t.e. it feeds, grows, and reproduces 
itself, and all these functions are performed by one 
and the same cell. Such a unicellular plant cannot 
properly be considered as strictly undifferentiated, 
since the permanent constituents of the cell, e.g. 
nucleus and chromatophore, must be regarded as defi- 
nite organs. 
As we pass from the unicellular plants to the simpler 
multicellular forms (see Fig. 9, F) we find the first 
indications of a specialization of certain cells. Thus the 
basal cell has very little chlorophyll, but is modified 
into a root-like organ for the attachment of the plant, 
while the other cells with their numerous chloroplasts 
are alone concerned with the nutrition of the plant. 
Very much more complicated are many of the large 
sea-weeds, some of which, like the great kelps (see 
Fig. 17), reach an enormous size. In these plants 
there are various sorts of cells aggregated into definite 
organs. The chloroplasts are mainly confined to the 
outer part of the plant, where they may be fully exposed 
to the light, while the inner tissue has little or no 
chlorophyll, and the cells are modified for conducting 
purposes. In the most highly organized of these marine 
alge, like the gulf-weed (Fig. 18), a further advance 
is seen in the formation of flattened leaves to which 
the chlorophyll-bearing cells are mainly restricted. 
Indeed these highly specialized sea-weeds bear a most 
remarkable superficial resemblance to the flowering 
plants in the development of a definite branching axis 
