Plant Kingdom (Marine Plants) Marine plants (primarily phytoplankton) form the base of the oceanic food chain. Because marine plants require sunlight for their vital processes, most growth intertidally to just over 100 feet. A few species grow deeper, and in extremely clear water of the Caribbean, some species may be found at surprising depths. There are over 600 species reported in the western tropical Atlantic. There are 2 basic types of marine plants. Flowering plants, have true roots (containing conductive tissue), stems, leaves and flowers. Algae (singular alga) have no true roots, but attach to the substrate by holdfast structures called rhizoids, and runners connecting upright blades, called rhizomes. The upright parts of the plant are called stalks rather than stems, and blades instead of leaves. Several species of algae have bulb-like structures containing gas, called bladders or floats, that keeps the structure upright. Most common marine algea can be visually identified by the shape of their blades and branching pattern. Algea are classified into Phyla on the basis of their predominate photosynthetic pigment. Although all contain some chlorophyll, only one group of algea is green. FLOWERING PLANTS (Class Angiospermae) There are only three species, and each can be easily distinguished by the size and shape of its leaves. BROWN ALGEA (Phylum Phaeophyta) Brown algea ranges in color from shades of pale creamy brown to yellow-brown, green-brown and dark brown. Occasionally they are marked with shades of yellow, green and blue. Color is primarily the result of a brown pigment called fucoxanthin. GREEN ALGEA (Phylum Chlorophyta) The most commonly observed algea on tropical reefs are green. Many are calcareous and along with coralline re algea add significant amounts of calcium carbonate to the reef. Chlorophyll is the pigment primarily responsible for their color. RED ALGEA (Phylum Rhodophyta) They are the most diversified of the algea, with over 4000 tropical species. Calcareous species are important and integral elements in the reef building process. Many of those adding calcium carbonate to the reef. Many noncoralline red algea are weed-like, growing in reddish translucent tangles, while other appear mossy or fuzzy. The primary red-pink pigment is phycoerythrin.