440 A TEXTBOOK OF BOTANY [Ch. X 



phyll, but of which the function is uncertain. Their bodies 

 are mostly leathery-tough and elastic, in correlation with 

 their exposure to the force of great breakers, and they are 

 strongly attached to rocks by firm holdfasts which suggest 

 roots in their aspect, but are quite different in nature. 

 These holdfasts are connected with the leaf-like fronds by 

 stalks, often of great length ; and many of the prominent kinds 

 possess floats, in the form of gas-holding bladders, which 

 raise the fronds towards or upon the surface. Unlike all 

 lower forms, which are filmy, or finely divided in correlation 

 with the conditions of gas absorption from the water (page 

 562), they possess somewhat massive fronds and stalks 

 often many cell layers thick; and they even exhibit some 

 differentiation of tissues^ — cortical, storage, and conduct- 

 ing, — suggestive of cellular differentiation in the higher 

 plants. Their considerable thickness seems clearly con- 

 nected with the effective aeration permitted their fronds by 

 the daily exposure directly to the air, either by the fall of 

 the water around them, or their flotation at low tide on the 

 surface. They do not admit air into intercellular spaces, 

 but the needful gases diffuse from cell to cell from the con- 

 stantly renewed supply at the surface ; and perhaps the dis- 

 tinctive brown pigment has connection with this function. 

 Somewhat over 1000 species are known, of which a few have 

 minor economic uses as fertilizers or sources of certain chem- 

 icals. They include two orders : 



Order 1 . Phmosporales : the Kelps and kin. 

 Order 2. Fucales: the Rockweeds. 



Order 1. Ph&osporales (Laminariales) : the Kelps 

 and kin. These are the largest of Algae, and include indeed 

 some of the longest of plants. The "Devil's Aprons" 

 (Laminaria species), of the Atlantic coast, growing just 

 below low tide mark and often cast upon the beaches, have 

 leathery fronds, 20 or 30 feet long and one or two broad, with 

 several feet of cylindrical stalk (Fig. 306). The largest 



