This group is characterized primarily by possession of a coenocytic structure in which true septa are rare or absent, the coenocyte normally having a cytoplasmic lining surrounding a central vacuole and containing numerous disc-shaped chloroplasts. The order can be subdivided into two divisions, the Eusiphoneae (Halicystidaceae, 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 establishment 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 Chlorophyceae as a whole. Most of the genera possess the power of regeneration 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.
KeywordsSexual Reproduction Central Vacuole Contractile Vacuole Zygote Nucleus Cellulose Layer
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