THE ALGAE 175 



The plants show a diplobiontic ahernation of generations and have the 

 most elaborate carposporophytes found in the Rhodophyceae, though the 

 type Corallina, which we shall consider in detail, is not the most highly 

 developed genus in this respect. 



Many of the Cryptonemiales have their cell walls impregnated with lime. 

 In some the original filamentous character of the thallus is retained, but in 

 others the whole thallus becomes so encrusted in lime that the plant appears to 

 be petrified and forms an amorphous mass resembling coral. Such calcareous 

 Algae do indeed contribute appreciably to the building up of coral reefs. 

 The genus Lithothamnion is particularly important in this respect. 



As a type of this Order we shall consider the common calcareous Alga, 

 Corallino officinalis. 



Corallina officinalis 



This species occurs very commonly on British shores between tide marks. 

 It is found especially in rock pools, where its small, reddish, jointed tufts 

 are very characteristic (Fig. 167) The colour of the plants varies according 



Fig. 167. — Corallitm officinalis. Habit of 

 growth of the plant. Natural size. 



to the depth of water in which it is growing. In very deep water it is 

 dull purple, becoming pink and finally white with increasing exposure. 

 The plants are small, rarely more than 9 cm. high, with a basal disc from 

 which the pinnately branched segmented axes arise. These axes are made 



