230 



STRUCTURE AND LIFE HISTORIES 



relatively narrow (from K to > inch), several feet in 

 length, and interrupted at frequent intervals by swellings 

 or nodes, which are air sacs, and add greatly to the buoy- 

 ancy of the plant in water (Fig. 172). Many of theFucacese 

 possess two kinds of branches, more or less distinct from each 

 other long branches and short branches, or spurs. This 

 is a phenomenon which occurs in several groups of plants, 



FIG. 173. Ascophyttum nodosum (L) Lejol. Radical longitudinal sec- 

 tion of an old branch of the thallus. c t cortical tissue, the seat of photo- 

 synthesis; m, central tissue, or medulla. (Redrawn from Reinke.) 



and notably in the pines, to be studied later. In Ascophyll- 

 um the distinction between long and short branches is not 

 as strongly marked as in some other forms, such, for ex- 

 ample, as Scaberia. The short branches have enlarged 

 tips, which somewhat resemble the swellings of the main 

 stem. The plant has a "rubbery" appearance, with a 

 smooth, slippery surface, and is usually attached to rocks 

 by a "hold-fast" organ. 



209. Anatomy. A study of the internal structure 

 (Fig. 173) reveals two systems of tissues, more or less 

 clearly distinct: 



