THE ALG.E 



183 



to each other. The cells may either round themselves 

 off and separate immediately after each division, or 

 may remain grouped together for a few generations. 

 Sooner or later, however, they fall apart. The plant 

 forms small starch-grains in 

 the chloroplasts when exposed 

 to light. 



We have in Pleurococcus an 

 example of a typically unicel- 

 lular plant, in which the cells 

 lead a perfectly independent 

 life ; each individual cell, how- 

 ever, has the same structure 

 as in higher plants. Evidence 

 has recently been brought FIG. 77. Pleurococcus vulgaris. 

 forward to show that Pleuro- A > sillgle cell; - 71 ' imcleus; 

 coccus possesses other means 

 of reproduction (zoospores and 

 sexual motile cells) besides 

 simple cell-division, and that 

 it may grow out into filaments 

 like those of higher green Algae. It thus appears that 

 the life-history is in reality a rather complex one, and 

 that the common unicellular condition is due to reduction 

 from a more advanced type. 



R THE BEOWN ALG^ (PliMpliycece) 



The brown Alg< r e, almost all of which are seaweeds, 

 are probably better known to the ordinary observer 

 than even the green group, owing to the large size 

 which many of them attain, and the extraordinary 

 abundance in which they occur on our coasts. In 

 dimensions and structure they present an even wider 



cli, chloroplastids. B, four 

 cells separating after division. 

 C, group of cells remaining in 

 contact. The two to the left 

 have just divided afresh. D, 

 tetrahedral group. Magnified 

 540. (After Strasburger.) 



