LAMINARIAS 



319 



plant body commonly consists of a stalk bearing leaf -like branches 

 and attached to a support by root-like holdfasts. One might 

 think such a plant too complex to be classed as a Thallophyte, 

 for, according to definition, a Thallophyte is a plant not differen- 

 tiated into roots, stem, and leaves. However, when the struc- 

 ture of these parts that so much resemble roots, stems, and leaves 

 is studied, one finds that they are too simple in structure to be 

 classed as such organs, although they mark a notable advance- 

 ment over the Green Algae in the differentiation of the plant body. 

 Some have special swollen regions called air bladders, which help 



FIG. 274. One of the Brown Algae, Macrocystis, showing the root-like 

 holdfasts, the stem-like axis, and the leaf -like blades. Much reduced. Re- 

 drawn with modifications from Harvey. 



the plant to float, and in connection with reproduction there is 

 much differentiation shown by some forms. 



There are about 1000 species of Brown Algae known and these 

 are divided into two groups, one of which comprises the Kelps 

 and closely related forms, and the other, the Rockweeds and Gulf- 

 weeds. 



Kelps and Closely Related Forms (Phaeosporales). This 

 order comprises a number of families of which the Laminarias or 

 Kelps are the largest forms. 



Laminarias. These are the largest of Algae, and include such 

 conspicuous forms as Nereocystis, Postelsia, or Sea Palm, and 

 the huge Macrocystis (Fig. 274), which is sometimes more than 

 200 feet in length. It is from the massive plant bodies of the 



