VIl] CAULERPACEAE. 157 



a. SIPHONEAE. 



Thallus consisting of simple or branched cells very rarely 

 divided by septa, and containing many nuclei. In certain 

 genera the branches form a pseudoparenchymatous tissue by 

 their repeated branching, and as a result of the intimate felting 

 together of the branched cells. Reproduction is effected either 

 by the conjugation of similar gametes or by the fertilisation 

 of an egg-cell. 



Vaucheria and Botrydium are two well-known British 

 genera of this order, but most of the recent representatives live 

 in tropical and sub-tropical seas. The most striking charac- 

 teristic feature of this division of the Chlorophyceae is the fact 

 that the thallus of a siphoneous alga consists of an unseptate 

 coenocyte ; the plant may be extremely small and simple, or it 

 may reach a length of several inches, but in all cases the body 

 does not consist of more than one cell or coenocyte. 



From a palaeontological standpoint the Siphoneae are 

 of exceptional interest. It is impossible to do more than refer 

 to a few of the living and fossil genera. There are numerous 

 fossil representatives already known, and there can be little 

 doubt that further research would be productive of valuable 

 results. 



As examples of the order, a few genera may be described 

 belonging to the three families Caulerpaceae, Codiaceae, and 

 Dasycladaceae. 



a. Caulerpaceae. 



Thallus unseptate, showing an extraordinary variation in 

 the external differentiation of the plant-body. Reproduction is 

 effected by means of detached portions of the parent plant. 



The genus Gaulerpa, represented by a few species in the 

 Mediterranean and by many tropical forms, has already been 

 alluded to as a striking example of a plant which appears 

 under a great many different forms \ As a recent writer 

 has said, "Nature seems to have shown in this genus the 

 utmost possibilities of the siphoneous thallus'." Fragments of 

 1 Vide p. 142. » Murray G. (95) p. 128. 



