230 
STRUCTURE AND LIFE HISTORIES 
relatively narrow (from y 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 theFucaceae 
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 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: 
