THALLOPHYTA: ALGAE 19 



yellow pigments, especially carotin, in their plastids. Xanthophijll and 

 carotin are the two carotinoid pigments associated with chlorophyll in 

 other green plants, but here their proportions are different. Although 

 a few are marine, most yellow-green algae are found in fresh water. 

 They are either unicellular or multicellular. Many are flagellates. The 

 group was formerly classified with the Chlorophyceae, but seems to have 

 had an independent origin from a flagellate ancestry and to have fol- 

 lowed a line of evolution parallel to that of the green algae. Three rep- 

 resentative genera are Chlorochromonas, Tribonema, and Botrydium. 



Chlorochromonas. This is a naked unicellular flagellate with two yel- 

 low-green plastids (Fig. 8). It has two cilia (flagella) of unequal length 

 attached anteriorly, a contractile vacuole, and a cen- . 



tral nucleus. It stores food as leucosin and probably 

 also as oil. A leucosin granule, contained in a vacuole, 

 lies at the posterior end of the cell. Reproduction 

 takes place by fission. From such a form as Chloro- 

 ckromonas, the other Xanthophyceae appear to have 

 evolved. 



Tribonema. This is a filamentous alga, widely 

 distributed in fresh-water pools (Fig. 9). The fila- 

 ments are unbranched and composed of elongated 

 cylindrical cells. The walls are made up of two over- 

 lapping pieces that appear H-shaped in a longitudinal 

 section. The cells contain a nucleus and a number of chromonas minuta, 

 yellow-green plastids. Asexual reproduction occurs x 2,000. {After 

 by the formation of aplanospores, akinetes, or zoo- 

 spores. Aplanospores are nonmotile spores with a wall distinct from the 

 wall of the parent cell. Akinetes are also nonmotile but are derived from 

 an entire vegetative cell whose wall becomes the wall of the spore. Zoo- 

 spores are ciliated and naked. In Tribonema one or more aplanospores 

 may be produced within a cell, while the zoospores are usually formed 

 singly. Sexual reproduction, which is rare, takes place by the fusion of 

 isogametes formed in ordinary cells. Usually one gamete settles down 

 before the other unites with it. The motile cells have two cilia of unequal 

 length, attached anteriorly, and the reserve food is stored as oil or leu- 

 cosin, never as starch. 



Botrydium. Botrydium is a terrestrial alga often found on wet muddy 

 flats. The vegetative body is unicellular and multinucleate, consisting 

 of a balloon-shaped bladder about 1 to 2 mm. in diameter (Fig. 10). It 

 is fastened to the soil by means of branched colorless rhizoids. The 

 cytoplasm, containing many nuclei and, in the aerial portion, numerous 

 yellow-green plastids, forms a thin layer lining the cell wall and enclosing 



