8o CHLOROPHYCEAE 



chloroplast that congregates with the cytoplasm at certain points in 

 the older plants and then each group is cut off by a membrane, 

 thus producing a number of marginal cells. This septation is 

 regarded as a primitive character that is slowly being lost because in 

 the more advanced Siphonales it is restricted to the formation of 

 the reproductive organs. The cells do not necessarily form a con- 

 tinuous layer and are frequently restricted to the basal region where 

 they may develop rhizoids, whilst in other species they are nearer to 

 the apex where they may give rise to proliferations. The lower cells 

 can form short creeping branches, and as these bear more of the 

 erect vesicles a tuft of plants is produced. One genus (Siphono- 

 cladus), classed either in the Valoniaceae or else in a separate group, 

 resembles Cladophora very closely although the method of seg- 

 mentation is essentially the same as that of Valonia. Reproduction 

 in Valonia takes place by means of bi- or quadriflagellate swarmers 

 which are liberated from the cells through several pores, and al- 

 though no sexual fusion has been seen as yet, nevertheless meiosis 

 occurs in V. utricularis at swarmer formation. The plants are 

 therefore diploid, a condition that is also characteristic of most of 

 the Siphonales. The reproductive cells may encyst themselves, and 

 it has been suggested on this evidence that the plant is a colonial 

 aggregate of coenocytic individuals resulting from the retention of 

 cysts which have developed in situ. The correctness or otherwise of 

 this interpretation can only be obtained through a better knowledge 

 of its phylogenetic history and the reproductive processes of other 

 members of the group. 



*Dasycladaceae : Dasycladus {dasy, hairy; cladus, branch). Fig. 



55- 

 The family Dasycladaceae is very ancient and was formerly 

 much more widely spread since sixty fossil genera are known 

 whilst there are only ten living to-day (cf. p. 269). Dasycladus 

 forms dense growths, up to 5 cm. in height, in shallow waters 

 where the plants are anchored by means of richly branched non- 

 septate rhizoids. The central axis bears dense whorls of profusely 

 branched laterals which are arranged alternately above each other. 

 The branches arise in whorls of four immediately below the apex 

 of the parent cell, to which they are united by narrow constrictions, 

 and although the rest of the main axis is impregnated with lime 



