Cladopkoracew 259 



developed from the basal segment of the thallus, but the older plants of some 

 genera break away from their attachment to the substratum and become 

 free-floating. 



Nearly all the forms are aquatic and most of them marine. A few are 

 epiphytic or even epizootic, and more than one species of Rhizoclonium lives 

 on damp soil. 



The growth of the thallus is apical in Cladophora and Pithophora, but 

 mostly intercalary in the other genera. Transverse walls are formed quite 

 independently of nuclear divisions and by the gradual ingrowth of a thick 

 ring-like septum (consult fig. 167 G F). 



In Acrosiphonia, in the sEgagropila-section. of Cladophora, and especially 

 in Pithophora, curious claw-like branches are sometimes developed. 



The protoplasm lines the interior of the wall of each segment and usually 

 contains many nuclei, although in the Rhizocloniese the number of nuclei 

 may be reduced to two or even one (in Spongomorpha). There may be a 

 single parietal, reticulated chloroplast, with many pyrenoids, or a number of 

 isolated chloroplasts, each with one pyrenoid, and all intermediate states 

 between these two extremes are of frequent occurrence. 



In the sEgagropila-section of Cladophora the thallus often becomes 

 fragmented by the death of the basal parts, each separate portion being able 

 to grow into a complete new thallus. In Acrosiphonia, Spongomorpha and 

 some species of Ch&tomorpha the rhizoids may form a pseudoparenchy- 

 matous tissue in which reserve materials are stored, and, should the ordinary 

 vegetative filaments die away, this tissue remains alive and subsequently 

 develops new filaments. Thick-walled ' ccenocysts ' of the nature of akinetes 

 are formed in Rhizoclonium (Wille ; Gay ; Brand) and Urospora from single 

 segments of the thallus. On germination the ccenocyst forms a new filament 

 in Rhizoclonium, but in Urospora it sometimes produces zoogonidia. Similar 

 ccenocysts occur in Pithophora, but in this genus they have a very special 

 origin. 



Asexual reproduction by zoogonidia is known in all the genera except 

 Pithophora. The zoogonidia are biciliated in Chtetomorpha, Rhizoclonium 

 and Cladophora, but quadriciliated in Urospora. 



Isogametes occur in Cladophora and ChcVtomorpha, and anisogametes in 

 Urospora. The zygotes germinate at once or after a period of rest. 



The thallus is more or less destitute of a mucous covering, with the result 

 that it is often loaded with smaller epiphytic AlgaB. The latter may belong 

 to the BacillarieaB (fig. 84), Myxophycese or ChlorophyceaB, and are sometimes 

 so thickly clustered as quite to obscure the host-plant. 



The Cladophoracea3 have a world-wide distribution in both salt and fresh 

 water. Pithophora is a freshwater genus mostly confined to tropical and 

 subtropical areas. 



172 



