GIGARTINALES 



239 



colour of their own as might be suspected from their parasitic 

 nature, and they send out branched filaments or haustoria into the 

 tissues of the host. The parasites appear as external cushions lying 

 on the branches of the host, each cushion, which is surrounded by 

 an outer gelatinous coat, consisting of a central area that is four to 

 five cells thick. In Holmsella the carpogonial branch is two-celled 

 whilst in Harveyella it is four-celled, this feature forming one of the 

 principal differences between them. The antheridial, carpogonial 

 and tetrasporic plants are all separate, and the species are said to 

 pass through the full floridean life cycle twice every year. It is 

 clear that their much-reduced morphological features are to be 

 associated with the parasitic habit, and have probably arisen as a 

 result of the adoption of parasitism. 



GiGARTiNACEAE : Choudrus (cartilage). Fig. 159. 



This is a widespread genus, many of the species appearing as a 

 number of varieties, some of which are probably only ecological 



Fig. 159. Chondrus crispiis. A, plant ( x f ). B, transverse section of thallus ( x 344). 

 (A, after Newton; B, after Kylin.) 



forms. Chondrus crispus, which is known as '' Irish moss", contains 

 80 % of water together with salts that control gelatinization. The 

 plants are often collected and bleached, and then an extract is 



