PH^EOSPORE^E 



245 



Wille (Sieve-tubes) Ber. Deutsch. Bot. Gesell., 1885, p. 29. 

 Gardiner (Conjugation of Zoospores) Proc. Cambr. Phil. Soc., 1886. 

 Humphrey (Agarum) Proc. Amer. Acad. Sc., 1886, p. 195. 

 Oliver (Sieve-tubes) Ann. of Bot., i., 1887, p. 95. 



In the PUNCTARIACE^E, SPOROCHNACE^E, and SCYTOSIPHONACE^E 

 the limits of which orders are not settled by systematists the structure 

 of the thallus varies greatly. In Punctaria (Grev.) and Phyllitis (Ktz.) 

 it is flat and leaf-like, from one to six layers of cells in thickness; in 

 other genera it is slender, cylindrical, erect, and more or less branched, 

 the main axis being either solid or hollow, and consisting of a 

 pseudo-parenchymatous tissue, in which the outermost or the two or 

 three outer rows of cells are much smaller than the inner ones. In 



FIG. 218. Asperoccccns bnllosus Lmx. a, natural size (after Bornet) ; , portion of surface with 

 sorus(x 100) ; c, transverse section through thallus and sorus ( x 100). (After Kiitzing.) 



Dictyosiphon (Grev.) the 'frond' branches into delicate hairs. In 

 Arthrocladia (Duby) the branches are arranged in delicate whorls. In 

 Scytosiphon (Ag.) the thallus is elongated, cylindrical, and unbranched, 

 but constricted at intervals, and resembles that of Chorda in habit. In 

 other genera, as Asperococcus (Lmx.) and Hydroclathrus (Bory), it is 

 hollow and bladdery. In most of the genera both kinds of zoosporange 

 are known, while in others one or the other has not yet been . detected. 



