196 



ALG^E 



near the margin of the ' frond,' sometimes (Rhodymenia bifida, Ktz.) im 

 bedded in the thallus, and then often grouped into sori (Nitophyllum,, 

 Grev.) ; or, in the Corallinaceae, enclosed in special conceptades. In 

 other genera (Phyllophora, Grev., &c.) they are developed in nematheces^ 

 wart-like elevations of the surface, where they are accompanied by barren 

 hyphae or paraphyses. In others again they are borne on metamorphosed 

 pod-like branches known as stichids, as in Dasya (Ag.), Plocamium (Lmx.), 

 &c. Only in the Porphyraceae are the tetraspores endowed with a slow 



FIG. 173. Xitophyllum punctatuin Harv. , piece of frond with tetrasporanges (natural size) ; b, piece 

 of the thallus with tetrasporanges (x 100) ; c, section through frond showing cystocarp(x 100 . 

 (After Kiitzing.) 



amoeboid motion. Zoospores are altogether unknown in the class, except 

 in the Ulvaceae ; but other modes of non-sexual propagation occur in a 

 few cases. In some genera of Ceramiaceae special organs occur, known 

 as seirospores. Melobesia is characterised by the production of genuine. 

 In Monospora (Sol.) stalked gemmae or propagules are produced at the 

 forks of the branchlets, and readily become detached, apparently repla- 

 cing the sexual organs, which are unknown in the genus. Lemanea 

 (Bory) increases by budding. In Hydrolapathum peculiar bud-like 

 prolifications are produced on the stem (see fig. 171). 



