238 THE ALGAE 



Hildenbrandtiaceae: Hildenhrandtia (after F. E. Hildenbrandt). 

 Fig. 132 

 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 frequendy 

 difficult to distinguish 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 forming indefinite patches that are at- 

 tached by a strongly adhering lower surface. The genus is both 

 marine and fresh water, Hildenhrandtia rivularis appearing fre- 

 quendy in rivers and streams. The principal mode of reproduction 

 is by means of tetraspores which are produced in sporangia borne 

 in rounded or oval conceptacles that are sunk in the thallus. The 



absence of sexual plants suggests 

 speciahzation, though it is possible 

 that a protonemal gametophyte 

 exists which has not yet been dis- 

 covered. Recentiy another genus, 

 Apophlaea of New Zealand, has 

 been added to this family. It is 

 characterized by a prostrate 

 stem from which short, erect, 

 stout, branched projections arise 



Fig. 132 HiMenbrandtia prototypus ^ ^ ^ tetrasporic COn- 



Tetraspores in conceptacles ( x 320). '■^^"'- '^'"" ^ 



(After Taylor.) ceptacles. 



Corallinaceae: Epilithon (epi, above, lithon, stone). Fig. 133 



This and the succeeding type belong to the Corallinaceae, a 

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

 building of rocks and coral reefs and which have been known as 

 fossils from the earhest geological strata. The present type has been 

 selected because the common species, E. membranaceum, 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. 



^ e.g. Lithothamniorii Lithophyllum. 



