330 



STRUCTURE AND LIFE HISTORIES 



in 



relatively narrow (from J^ to J-^ inch), several feet 

 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 the Fucaceae 

 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. — Ascophyllum nodosum (L) Lejol. Radical longitudinal sec- 

 tion of an old branch of the thallus. c, 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 Scaheria. 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: 



