226 



RHODOPHYCEAE 



SQUAMARiACEAE:Hildefibrandtia{2ifterF. E. Hildenbrandt). Fig. 149. 



This genus is characteristic of a small group of red algae all of 

 which form thin crusts on stones or other 

 algae, and it is frequently difficult to dis- 

 tinguish in the field from similar encrusting 

 brown types such as Ralfsia. The frond is 

 horizontally expanded into a thin encrusting 

 layer composed of several layers of cells 

 arranged in vertical rows, the plants form- 

 ing indefinite patches that are attached by 

 a strongly adhering lower surface. The 

 genus is both marine and fresh water, 

 Hildenbrandtia rivularis appearing fre- 

 quently in rivers and streams. The principal 



mode of reproduction is by means of ^^g- ^49- Hildenbrandtia 



\ . , II- • prototypus. Tetraspores in 



tetraspores which are produced m sporangia conceptacles ( x 320). (After 



borne in rounded or oval conceptacles that Taylor.) 

 are sunk in the thallus. 



*CoRALLiNACEAE : EpiUthon (epi, above, lithon, stone). Fig. 150. 



This and the succeeding type belong to the Corallinaceae, a family 

 of calcareous red algae which have played much part in the building 

 up of rocks and coral reefs and which have been known as fossils 

 from the earliest geological strata. The present type has been 

 selected because the common species, E. mefnbranaceum, is less 

 calcified than other members of the Corallinaceae and thus forms 

 very convenient material for sectioning and demonstration pur- 

 poses without the trouble of decalcification. The thallus, which 

 forms a crust on other algae or phanerogams, consists of a single 

 cell layer composed of large cells, from each of which is cut off a 

 small upper cell that goes to form the outer lime-encrusted layer. 



Further divisions take place internally from the large basal cells 

 so that one finally obtains rows of erect filaments growing side by 

 side. The various reproductive organs are borne in conceptacles 

 on separate plants; in the male plants, for example, there are a 

 number of two-celled filaments in the centre of every conceptacle. 

 The basal cells of these threads cut off two antheridial mother cells 

 which in their turn produce two antheridia, whilst the upper cells 

 grow out to form the walls of the conceptacle. In the female plant 



