84 THE ALGAE 



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

 Figs. 45, 46 

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

 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-Uving, 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, containing 

 numerous pyrenoids, are arranged parietally with processes pro- 

 jecting 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 disc 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 accotmts 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. In C. verticillata branching is more or less restricted 

 to the base and the branches are long and simple like a Chaeto- 

 morpha thread. 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 sub- 

 stances or folded lamellae. Additional supporting rhizoids usually 

 develop from the basal and subbasal cells of the lower branches. 



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

 {h) 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 Lake Soro the water in April and May is 

 sufiiciendy free of diatoms for light to penetrate to such an extent 

 that photosynthesis increases and so much gas collects in the centre 

 of these balls that they float to the surface. Their characteristic 



