ocarpus_ (Fig. 28, _A_), are simple, branched filaments, but most
are large plants with complex tissues. Of the latter, a familiar
example is the common kelp, "devil's apron" (_Laminaria_), often three
to four metres in length, with a stout stalk, provided with root-like
organs, by which it is firmly fastened. Above, it expands into a
broad, leaf-like frond, which in some species is divided into strips.
Related to the kelps is the giant kelp of the Pacific (_Macrocystis_),
which is said sometimes to reach a length of three hundred metres.

[Illustration: FIG. 28.--Forms of brown seaweeds. _A_, _Ectocarpus_,
x 50. Sporangia (_sp._). _B_, a single sporangium, x 150. _C_, kelp
(_Laminaria_), x 1/8. _D_, _E_, gulf weed (_Sargassum_). _D_, one-half
natural size. _E_, natural size. _v_, air bladders. _x_, conceptacle
bearing branches.]

The highest of the class are the gulf weeds (_Sargassum_), plants of
the warmer seas, but one species of which is found from Cape Cod
southward (Fig. 28, _D_, _E_). These plants possess distinct stems and
leaves, and there are stalked air bladders, looking like berries,
giving the plant a striking resemblance to the higher land plants.

CHAPTER VII.

CLASS III.--THE RED ALGAE (_Rhodophyceae_).

These are among the most beautiful and interesting members of the
plant kingdom, both on account of their beautiful colors and the
exquisitely graceful forms exhibited by many of them. Unfortunately
for inland students they are, with few exceptions, confined to salt
water, and consequently fresh material is not available. Nevertheless,
enough can be done with dried material to get a good idea of their
general appearance, and the fruiting plants can be readily preserved
in strong alcohol. Specimens, simply dried, may be kept for an
indefinite period, and on being placed in water will assume perfectly
the appearance of the living plants. Prolonged exposure, however, to
the action of fresh water extracts the red pigment that gives them
their characteristic color. This pigment is found in the chlorophyll
bodies, and usually quite conceals the chlorophyll, which, however,
becomes evident so soon as the red pigment is removed.

The red seaweeds differ much in the complexity of the plant body, but
all agree in the presence of the red pigment, and, at least in the
main, in their reproduction. The simpler ones consist of rows of
cells, usually branching like _Cladophora_; others form cell plates
c