160 Plant Biology 



cell dies. The cell walls are ornamented with patterns of fine dots, or 

 perforations, which create beautiful designs unique for the various species. 

 Each protoplast of the cell contains one or many yello\vish-brown plas- 

 tids which impart the brownish color to most diatoms, although some 

 species have green or blue plastids. Each cell contains an organized 

 nucleus. Reserve foods include fats and an insoluble food known as 

 volutin (vo-lu'tin). 



Reproduction may occur asexually by cell division with each new cell 

 formins: a new valve inside the old one. Eventuallv certain of these 

 cells will become smaller and smaller. In the latter case certain rejuve- 

 nescent cells called auxospores (ok' so spor) (Gr. auxe, grow; spora, 

 spore) are produced. The latter usually result from the fusion of two 

 diatoms (gametes), and eventually a cell of normal size will be produced 

 again. The gametes are usually of equal size {isogamous) . 



Diatoms are common in fresh and salt ^vaters, although some species 

 are found in soils, on other plants, and even in hot springs. Diatoms are 

 important components of the diets of aquatic animals. When diatoms 

 die, their siliceous shells accumulate on the bottom to form diatomaceous 

 earth. The latter is used in preparing polishes, tooth powders, filters, 

 insulatins; materials, etc. It is believed that diatoms mav have aided in 

 the formation of oil because they are often found to be associated with 

 oil deposits in the earth. 



BROWN ALGAE (PHYLUM PHAEOPHYTA) 



The brown algae are multicellular and nonmotile, being attached by 

 rootlike holdfasts. Depending on the species, the plant body may be 

 composed of only a few cells or it may be over a hundred feet in length, 

 as in some of the kelps. They are marine and usually found in colder 

 waters. The chlorophyll is masked by the golden-brown pigment, fuco- 

 xanthin (fu ko -zan' thin) (L. fucus, alga or seaweed; xanthos, yellow). 

 Usually there are several plastids per cell, but no pyrenoids are present. 

 Each cell has a single nucleus and vacuoles. The cells have an organiza- 

 tion similar to that of higher plants, some e\en having a centrosome simi- 

 lar to the centrosome of animal cells. Stored foods are fats and soluble 

 sugars. 



All brown algae possess alternation of generations or metagenesis (met- 

 a-jen'esis) (Gr. meta, over; genesis, origin), in which a free-livang, 

 multicellular, gamete-producing gamctophyte (gam -me' to fite) (Gr. 



