SIPHONALES 91 



(ii) In more sheltered habitats the shoots are longer and more 

 branched. 



(iii) In deep water the plants are very large with richly branched 

 flabellate shoots. 



There is no septation, but the coenocyte is traversed instead by 

 numerous cylindrical skeletal strands, or traheculae, arranged 

 perpendicularly to the surface and which are most highly de- 

 veloped in the rhizomes. They arise from rows of structures termed 

 microsomes, and are at first either free in the interior of the coenocyte 

 or else connected with the wall, although in the adult state they are 

 always fused to the walls. The function of the trabeculae, which 

 increase in thickness at the same time as the walls by successive 

 deposition of callose, is extremely problematical and may be 



{a) mechanical: in this case they would presumably provide 

 resistance to high turgor pressures, although the presence of high 

 osmotic pressures in the cells has yet to be proved ; 



{h) to enlarge the protoplasmic surface ; 



{c) concerned with diffusion, because movement of mineral salts is 

 more rapid through these strands than through the cytoplasm; 



{d) lost or without any function. 



In addition to the trabeculae there are also internal peg-like 

 projections. Vegetative reproduction occurs through the dying 

 away of portions of the old rhizome thus leaving a number of 

 separate plants. The swarmers or gametes are formed in the aerial 

 portions and are liberated through special papillae that develop on 

 the frond. The sexual reproductive fronds have a variegated 

 appearance caused by the massing of the biflagellate gametes at the 

 different points, the swarmers in some species being separable into 

 micro- and macrogametes. In certain species the whole plant can 

 produce swarmers, whilst in others the reproductive area is limited, 

 and in such cases the morphological identity and differentiation of 

 the frond becomes of great interest. The thallus can be regarded 

 as composed of a number of individual cells which only become 

 evident at gametogenesis. Fusion between the swarmers has been 

 observed in C. racemosa, and it is probable that in all the species the 

 motile bodies are functional gametes and that the adult plants are 

 diploid. The genus has been much employed in experiments on 

 polarity because the structure of the thallus renders it extremely 

 suitable. 



