Simple Plants With Chlorophyll — Algae 159 



Other reproductive cells are formed in gametangia (gam e -tan' ji a) (Gr. 

 gametes, spouse; angos, vessel) and each contains eight, sixteen, thirty- 

 two, or sixty- four gametes (sex cells). Each gamete is smaller than the 

 zoospore and bears only two flagella. The fusion of these gametes of 

 equal size {isogamy) produces a zygote, the gametes arising from differ- 

 ent filaments. Each zygote eventually will produce four zoospores, each 

 of which will attach and form a new filament by cell division. 



Desmids (des'mid) (Gr. desmos, chain). — These gree7i algae are 

 frequently found floating in fresh water and may be solitary or in fila- 

 mentous or irregular colonies. In most species each cell is divided into 

 two halves which are joined by a connecting isthmus (Fig. 30). Each 

 half cell contains a chloroplast. An organized nucleus is located in the 

 isthmus. Reproduction is by cell division and by conjugation, which 

 resembles the similar phenomenon in S pirogyra. 



YELLOW-GREEN ALGAE, GOLDEN-BROWN ALGAE, AND 

 DIATOMS (PHYLUM CHRYSOPHYTA) 



The yellow, or brown, carotinoid pigments are more abundant than 

 the chlorophyll so these algae have a golden-brown, or yellowish-green, 

 color. All pigments are contained in organized plastids. The cell walls 

 are usually composed of a pair of overlapping halves (valves) which are 

 frequently impregnated with silica (glasslike) . Depending on the species, 

 these algae may be unicellular, colonial, or, in a few instances, multicellu- 

 lar individuals. Stored foods are oils and an insoluble carbohydrate 

 called leucosin (lu'kosin) (Gr. leukos, white). The nucleus is well 

 organized. 



Reproduction occurs asexually by cell division, by m.otile zoospores, or 

 by nonmotile spores. Sexual reproduction, when present, is isogamous 

 (gametes which are alike) . The phylum contains the yellow-green algae, 

 the golden-brown algae, and the diatoms. About 5,700 species are classi- 

 fied in 300 genera. Various species of diatoms will be considered as typi- 

 cal forms. 



Diatoms (di' atoms) (Gr. dia, across or two; tome, to cut). — These 

 unicellular, delicate algae are common in fresh and salt water and may 

 form filaments or other types of colonies. Different species vary in shape, 

 including rods, disks, triangles, etc. (Fig. 31). Each cell is composed of 

 two overlapping halves (valves) like a pill box. The cell walls are trans- 

 parent and glasslike (siliceous) and do not disintegrate even when the 



