APPENDIX A 



601 



nucleus and plastids, containing chlorophyll and yellow and brown pigments, and 

 storing their food as fats. Their most distinctive feature is the silicious shell which 

 they secrete. It is made up of two valves that fit together like the halves of a pill 

 box, with the edges overlapping, and the surfaces of the valves have a remarkable 

 appearance of fine sculpturing or striation due to the presence of thick and thin 

 places in the wall. Diatoms are abundant in salt and fresh waters, both in the 

 free-floating plankton and attached to or resting on the surfaces of plants and 

 other objects. They are preponderant in the plankton of flowing streams. In 

 the ocean they are the most important of all the "base industry" organisms. 



The Brown Algae (Phaeophyceae) are plants with cells containing definite 

 nuclei and plastids, chlorophyll, and a yellowish-brown pigment that masks the 

 green color; they store their foods as carbohydrates and complex alcohols. They 

 are, with rare exceptions, marine and include the largest of all algae besides many 



Fig. A. 3. Representative diatoms. {Courtesy General Biological Supply House, Inc.) 



small forms (Fig. 26.6). Most of them occur in the cold and temperate zones. They 

 usually grow attached to rocks, with the long, limp thallus floating upward. The 

 principal advance of this group over the green algae is that the thallus shows the 

 beginnings of cell differentiation and division of labor. It is usually divided into a 

 rootlike or disklike holdfast attached to some object, a stem, and leaflike or ribbon- 

 like blades. These structures are only superficially like the roots, stem, and leaves 

 of higher plants, and the blades consist of semi-independent cells, as in other 

 algae. The gulf weed (Sargassum) is a drifting warm-water species of this group; 

 other common types are bladder wrack, or rockweed (Fucus), devil's apron 

 (Laminaria), and the large kelps of the Pacific Ocean, one of which (bladder 

 kelp, Nereocystis) may attain a length of 150 feet. About 900 species of brown 

 algae are known. 



The Red Algae (Rhodophyceae) are, like the last group, principally marine. 

 They occur for the most part in the temperate and tropical seas and are abundant 

 in coral reefs, to the formation of which some of them contribute. Indeed, some 

 of the red algae of the reefs are commonly mistaken for corals. The plants of this 

 group (Fig. 26.5) have cells containing definite nuclei, chlorophyll, and red and 

 blue pigments that mask the green of the chlorophyll ; they store their food prod- 

 ucts as a special type of starch. About 2,500 species are known. 



