74 CHLOROPHYCEAE 



demarcated from the other groups and any affinities would seem to 

 be somewhat distant. 



*Cladophoraceae : Cladophora {dado, branch; phora, bearing). 

 Figs. 51, 52. 

 This is a widespread genus that occurs in both fresh and saHne 

 waters. The sessile forms are attached by means of branched 

 septate rhizoids, but some of them (e.g. C fracta) may become 

 free-living later, whilst there is one complete section (Aegagropila) 

 which is wholly free-living, the species existing as ball-like growths. 

 The Cladophora thallus is composed of branched septate filaments, 

 each cell usually being multinucleate, though cells with one nucleus 

 have been recorded. The elongate reticulate chloroplasts, contain- 

 ing numerous pyrenoids, are arranged parietally with processes 

 projecting into the central vacuole, but under some conditions 

 they break up into fragments. There would not appear to be much 

 present support for the old view that the chloroplast of each cell 

 is a complex of numerous disk chloroplasts. The cell walls exhibit 

 stratification as they are composed of three layers, an inner zone, 

 a median pectic zone, and an outer zone which is said to be 

 chitinous. There is very little production of mucilage, and this 

 probably accounts for the dense epiphytic flora that is frequently 

 found associated with species of this genus. The branches arise 

 towards the upper end of a cell and later on are frequently pushed 

 farther up, a process known as evection, thus giving the appearance 

 of a dichotomy. All the cells are capable of growth and this is 

 especially evident in cases of injury, but normally most of the 

 plant growth is apical and in the section Aegagropila is wholly 

 confined to the apex. At cell division the new septa arise from the outer 

 layers and develop inwards, leaving in the process triangular-shaped 

 spaces which later on may become filled with pectic substances or 

 folded lamellae. Additional supporting rhizoids usually develop 

 from the basal and subbasal cells of the lowest branches. 



In the Aegagropila group the species can exist as (a) threads, (b) 

 cushions and (c) balls. The destruction of the old threads in the centre 

 of the ball results in a cavity which may become filled with water, gas 

 or mud. In Lac Soro the water in April and May is sufficiently free 

 of diatoms for light to penetrate to such an extent that photo- 

 synthesis increases and so much gas collects in the centre of these 



