Chapter V 



EUPHYCOPHYTA 

 CHLOROPHYCEAE {continued) 



SIPHONALES5 CONJUGALES5 CHAROPHYCEAE 



* SiPHONALES 



This group is characterized primarily by possession of a coeno- 

 cytic structure in which true septa are rare or absent, the coenocyte 

 normally having a cytoplasmic hning surrounding a central vacuole 

 and containing numerous disc-shaped chloroplasts. The order can 

 be subdivided into two divisions, the Eusiphoneae (HaUcystida- 

 ceae, Bryopsidaceae, Codiaceae) where the membrane contains 

 mannan and there is only one type of plastid, and the Caulerpeae 

 (Caulerpaceae, Udoteaceae) where the cellulose of the membrane 

 is replaced by zylan and where the thallus contains photosynthetic 

 chloroplasts and amylogenic leucoplasts. Some workers suggest 

 that the difference is sufficient to justify the estabUshment of 

 separate orders. The order may be polyphyletic in origin, and the 

 fact that it reaches its maximum development in warm waters is 

 possibly significant, not only in respect of the phylogeny of the 

 group itself, but also in considering the evolution of the Chloro- 

 phyceae as a whole. Most of the genera possess the power of re- 

 generation to a marked degree, but this can perhaps be regarded as 

 a primitive character that has persisted throughout the course of 

 time. In all cases so far studied the thallus is diploid and it seems 

 that the function of the haploid generation (sexual reproduction) 

 has been transferred to it. In those cases where it is known, sexual 

 reproduction is anisogamous. In some of the tropical genera, e.g. 

 Halimeda, there is intensive lime deposition. Representatives of the 

 order are known to have existed from very early times (see p. 297). 



Halicystidaceae : Halicystis {halt, salt; cystis, bladder) and Der- 

 hesia (after A. Derbes). Fig. 54 



The gametophytic plants consist of an oval vesicle, up to 3 cm. 



96 



