Nearly all of the marine plants fall into this botanical division, which is made up of primitive plants in which the body shows little or no differentiation of vegetative organs—that is, no true root, stem, or leaf. Important among these thallus plants are the marine algae and the marine fungi, especially the bacteria. Since bacteria constitute the subject of a more specialized study of the sea, they will be dealt with under a special heading in chapter XVIII.
Most algae are beautifully colored, and sometimes also iridescent. The pigments of the chromatophores intercept solar energy, which is used in the synthesis of organic compounds. The type of pigment or pigment combination occurring in the algae as color manifestations has led to the names commonly used for the classes:
Blue-green algae (Myxophyceae)
Green algae (Chlorophyceae)
Brown algae (Phaeophyceae)
Red algae (Rhodophyceae)
Yellow-green algae (a heterogeneous group variously classified by different authors)
In general, the colors are characteristic of the classes, but other characteristics associated with cell structure and life history are more fundamental in distinguishing the five groups. Each group has a considerable variation in general morphology, some features of which will be pointed out in a review of the classes. The first four, with the exception of some blue-greens, are attached plants, while the yellow-greens are characteristically floating, or planktonic, forms.