24 



OUTLINES OF PLANT LIFE. 



Polysiphonia. 



31. External form. — The body oi Polysiphonia, a slender 

 alga (fig. 24) which grows in abundance upon rocky sea- 

 coasts, , is much branched. The main axis is 

 made up in its larger parts of five or more rows 

 of cells, the central row being surrounded by a 

 jacket of at least four others (fig. 25); but at 

 the tips even of the main axis there is only 

 a single row of cells, as in the simflest algse 

 (fig. 26). The body of Polysiphonia, there- 



FiG, 24. Fig. 25. Fig. 26. 



Fiu, 24.— An entire plant of Polysiphonia^ showing mode of branching. Natural size. 

 — After Kiitzing. 



Fig. 25. — Transverse section of one of the branches of Folysipkonia, showing a 

 minute central cell with four large and four small cells surrounding it. Magnified 

 about 50 diam. — From a drawing by Mr. Grant Smith. 



Fig. 26.— Apex of a branch of Polysiphonia which has nearly ceased growing. Mag- 

 nified about 100 diam. — From a drawing by Miss Rowan. 



fore, is one of the simplest forms composed of cells massed 

 together. 



32. Growth. — Growth in length can take place only at the 

 ends of the main axis and its branches, because there each 

 apical cell (fig. 26) produces, by division near its base, the 

 new cells whose later division and enlargement make the 

 mature axes. 



33. Color. — In this plant, as in very many of the marine 

 algse, there is present, in addition to the green of the chloro- 

 plasts, a special red coloring matter. To the naked eye, this 



