258 



Siph onodadiales 



branching has none of the regularity found in that genus. In Microdictyon 

 the segments of the branches are more or less of the same size, and they 

 form a close but irregular network. In Anadyomene the septate branches 

 have long and short segments, and there are no interspaces between the 

 branches. Moreover, in this genus the thallus may be clothed on both sides 

 with an additional layer of closely united, isodiametric branch-segments. In 

 Microdictyon the apices of neighbouring branches fix themselves to neigh- 

 bouring segments and in order to strengthen their attachment a ring-like 

 thickening of cellulose is formed at these points. In Anadyomene such a 

 thickening is also found at the apex of each segment. 



The chloroplasts are small polygonal plates, joined by prolongations of 

 their angles in the young segments, but mostly isolated in older segments. 



Fig. 166. Microdictyon Montagneanum Gray. A, nat. size. B, a piece of the periphery of 

 the thallus showing the branches, x 20. (After Montagne, from Wille.) 



Division of the segments takes place much as in the Cladophoraceae, and 

 lentiform cells do not occur. 



The genera are : Anadyomene Lamouroux, 1812 [inclus. Cystodictyon Gray, 1866] ; 

 Microdictyon Decaisne, 1832 ; Rhipidiphyllon Heydrich, 1894. They are mostly tropical 

 and subtropical, occurring both in shallow water and down to a depth of 50 metres. 



Family Cladophoracese. 



The septate thallus in this family consists of cylindrical or somewhat 

 tumid coenocytes forming simple or branched filaments which are never 

 bound together by tenacula. The different genera of the family illustrate 

 every condition intermediate between simple filaments, such as those of 

 Chtetomorpha, and the richly branched species of Cladophora. In Urospora 

 there are short lateral outgrowths of the segments which are not separated 

 by walls, and in the various species of Rhizoclonium there occur short 

 unseptate or septate branches, generally of an irregular character. In 

 the branched forms the thallus is tufted, or in the sEgagropila-section. of 

 Cladophora the branches are so arranged that they form a compact spherical 

 or cushion-like mass. 



There is always a basal attachment, generally by one or more 'holdfasts 



