EUPHYCOPHYTA 83 



also possess the short rhizoids characteristic of Rhizoclonium. The 

 branches are each so long that they can also be mistaken for species 

 of Chaetomorpha. On account of its characters this genus appears 

 to occupy an important phylogenetic position (see p. 31 1). 



Cladophoraceae : Rhizoclonium (rhizo, root; clonium, branch). 

 Fig. 44 



The species are found in marine, brackish or fresh waters, 

 several being very abundant on sand or mud flats. The uniseriate 

 filaments possess occasional or numerous short septate or non- 

 septate colourless rhizoidal branches. The number of nuclei per 



Fig, 44 Rhizoclonium. A, part of filaments of R. riparium ( x 90). 



B, cell of R. hieroglyphicum to show structure of chloroplast. (A, 



after Taylor; B, after Fritsch.) 



cell ranges from 1-4, the actual number being related to the volume 

 of the cell, though it may also apparently be influenced by the 

 presence of epiphytes. Asexual reproduction is by means of bi- 

 flagellate zoospores which in some species are said to possess un- 

 equal flagella. Anisogamy similar to that of the related genus 

 Urospora has been recorded for R. luhricum, Urospora is of interest 

 because the zygote first produces a Codiolum stage (so called after 

 the alga it resembles), which is considered to be diploid, and this 

 gives rise to zoospores from which the normal filaments develop. 

 If this interpretation is correct we have a rare example within the 

 Chlorophyceae of alternation of morphologically dissimilar genera- 

 tions (see also below — Spongomorpha and Halicystis (p. 96)). 



