230 RHODOPHYCEAE 



possess a very conspicuous mid-rib with both macro- and micro- 

 scopic veins and they form magnificent plants for pressing as 

 herbarium specimens. The complex nature of the laciniate or 

 branched thallus can be seen from the figure. There are three 

 orders of cells with considerable intercalary division, although the 

 cortication of the primary cell filaments to form the veins does not 

 involve intercalary division. The cells of the thallus also become 

 united by means of secondary protoplasmic threads and they may 

 also develop thin rhizoids. The cystocarps are small stalked bodies 

 which are borne on the mid-rib, whilst the tetrasporangia are 

 produced in special fertile leaflets that arise from the mid-rib, but 

 as these do not possess the power of intercalary growth they differ 

 slightly in structure from the vegetative thallus. In the related 

 genus Martensia each tetraspore mother cell is multinucleate, 

 containing about fifty nuclei all of which degenerate except for one, 

 and from this the four nuclei of the tetraspores are produced. 



"RnoTiOMELkCEKE: Janczewskia (after E. de Janczewski). Fig. 153. 



This is a remarkable hemi- or holo-parasitic genus which is 

 always to be found on other members (Laurencia, Chondria and 

 Cladhymenia) of the same family. One of the most interesting 

 features of this parasitism is that the genus is very closely related to 

 Laurencia and yet is parasitic upon various species of that genus. 

 All the species have organs of contact or penetration, the latter 

 being fungal-like filaments which establish pit connexions with the 

 cells of the host. Each individual plant is a coalescent tubercular 

 mass composed of fused branches that grow from an apical cell 

 buried in a pit as in Laurencia. The sexual plants are dioecious and 

 the diploid asexual plant also occurs. 



*Rhodomelaceae : Polysiphonia {poly, many; siphonia, siphons). 

 Fig. 154- 

 The thallus in this genus generally arises from decumbent basal 

 filaments that are attached to the substrate by means of small 

 flattened disks. 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 



