234 ESSENTIALS OF BOTANY 



a thousand feet. The structure of some of the smaller 

 forms is not very unlike that of the pond-scums (Sect. 278), 

 while that of the larger ones, as the rockweeds, Fucus 

 (Sects. 295, 296), and the kelps is rather complicated. The 

 gulf-weed, Sargassum, related to the rockweeds, has ex- 

 panded leaf-like branches for photosynthesis, globular air- 

 bladders to float the plant, and small branches specialized 

 for reproduction. 



In the brown algge the coloring matter appears yellowish 

 brown or dark brown ; by washing the plants in fresh 

 water they soon become green, as the brown pigment is 

 dissolved and removed. The reproduction is of various 

 types, in the kelps wholly asexual, and in Fucus and its 

 allies wholly sexual (Sect. 298). 



RED ALG^i 



300. Occurrence. — The red algse are mostly marine and 

 many of them are familiarly known as sea-mosses. They 

 are especially abundant in the warmer seas, and some of 

 them flourish in deeper waters than are frequented by the 

 brown algse. 



301. Form and Structure. — Most of the red algse are of 

 moderate size, but some genera are minute, almost micro- 

 scopic. The simplest forms consist of delicate branching 

 threads, each composed of a single row of cells. Others, 

 like the so-called Irish moss, appear as rather stout branch- 

 ing thalli, and others form broad wavy sheets. Some occur 

 as incrustations on rocks or coral. The delicate feathery 



1 No laboratory studies on this group are given since the vegetative char- 

 acters are not important for study in a brief course, and the sexual reproduc- 

 tion is too complicated to be mastered by beginners in botany. 



