250 THE ALGAE 



flattened discs. These basal filaments do not represent a primary 

 heterotrichous condition but a secondary state. Many species are 

 epiphytic on other algae whilst P.fastigiata, which is always found 

 on the fronds of the fucoid Ascophyllum nodosum^ is probably a 

 hemi-parasite. The thallus is laterally or dichotomously branched 

 and bears numerous branches which are shed annually in the 

 perennial forms before winter and are redeveloped in the spring. 

 The main axes and branches are corticate or ecorticate, and possess 

 a polysiphonous appearance due to the single axial cell series being 

 surrounded by four to twenty-four pericentral cells or siphons. 

 The primary pericentral siphons represent potential branches and 

 are united to the parent axial cell by a pit connexion. The corticat- 

 ing cells, when present, are always shorter and smaller and are 

 often only found in the basal portions of the stem. The ultimate 

 branches are not polysiphonous and frequently terminate in delicate 

 multicellular hairs. 



The colourless antheridia, which are formed in clusters, are 

 borne on a short stalk that morphologically is a rudimentary hair. 

 In Polysiphonia violacea the two basal cells of the hair are sterile, 

 the upper one giving rise to a fertile polysiphonous branch and a 

 sterile hair. One or more mother cells are formed from all the peri- 

 central cells on the fertile branch, and each mother cell produces 

 four antheridia in two opposite and decussate pairs, the first and 

 third appearing before the second and fourth. There is no second- 

 ary crop in this species. The carpogonial branches are also formed 

 from hair rudiments, the support cell cutting off a small section 

 from which two lateral sterile cells arise. The second *sterile' cell 

 is morphologically equivalent to an abortive carpogonium. Later 

 on a fertile pericentral cell is cut off, and this gives rise to the four- 

 celled carpogonial branch, the carpogonium being of interest be- 

 cause there is a persistent nucleus in the trichogyne. 



After fertiUzation has taken place the auxiliary cell is cut off from 

 the apex of the support cell (Fig. 141, h) and in addition two branch 

 systems composed of nutrient cells develop from the original 

 sterile cells. When the zygote nucleus has divided, commonly only 

 one of the daughter nuclei passes into the auxiliary cell, which in 

 the meantime has become fused to the carpogonium and a new 

 wall is then laid down, cutting off the carpogonium. The auxiliary 

 cell next fuses with the pericentral cell and after the diploid nucleus 

 has been transferred it unites with the other support and axial cells 



