79 



pinnately branched, which, at the nodes, or throughout its entire 

 length, is covered by a layer of small cortical cells. Polysiphonia 

 (Fig. 74) has a filamentous, much branched thallus, made up of 

 a central cylindrical cell, surrounded by a layer of other cells, 

 cortical cells, which in length and position correspond to the 

 central ones. In many of the Red Alga? the vegetative organs 

 are differentiated into stems and leaves, the former having, as in 

 Chara, unlimited growth in length, whilst the latter soon attain 

 their full development. Choiidrus has a fleshy, gelatinous thallus, 

 without nodes ; it is repeatedly forked into flat branches of vary- 

 ing thickness. Farcellaria has a forked thallus with thick branches 



FIG. 73. CaUitJmmnion elegans : a a plant with 

 tetraspores ( x 20) ; b apex of a branch with 

 tetraspores ( x 250). 



Fis. 74. Polj/si'pTioni'a variegata : 

 a a portion of a male plant with 

 aiitheridia; b spermatia ; c trans- 

 verse section of thallus. 



and without nodes. The thallus of Delesseria (Fig. 76) consists of 

 branches, often bearing leaf-like structures, with a midrib and 

 lateral ribs springing from it. These ribs persist through the 

 winter, and at the commencement of the succeeding period of 

 vegetation the lateral ribs become the starting points for new- 

 leaves. In Corallina the thallus is pinnately branched, and 

 divided into nodes and internodes. The name has been given to 

 this genus from the fact that the thallus is incrusted with car r 

 bonate of lime to such a degree that it becomes, very hard, and the 



