248 THE ALGAE 



corticating threads develop from lateral cells which are cut off from 

 each segment just behind the apex. The male plants, which are 

 rare in nature, bear the antheridial sori on the upper regions where 

 they form whitish patches. A system of branching threads, which 

 appears as a preUminary to sorus formation, arises from a single 

 central cell, and from each of these branching threads two to three 

 antheridial mother cells grow out and increase in length. Depend- 

 ing on the species, one, two or three primary antheridia arise from 

 each mother cell and they may be followed by a crop of secondary 

 antheridia. The procarp consists of a support cell with a three- 

 celled carpogonial branch, both these and the antheridial mother 

 cells being uninucleate, although the mature vegetative cells are 

 multinucleate. There are one or two auxiliary cells, and after fertiH- 

 zation only one of these receives a process from the carpogonium 

 which carries with it the diploid nucleus. This auxiHary cell then 

 proceeds to cut off a segment on the outer side, and from this a 

 group of cells develops that ultimately gives rise to the gonimo- 

 blasts. The ripe cystocarps are sessile on the thallus and possess a 

 basal placenta. The tetrasporangia are borne on the diploid plants 

 in small cavities produced by the infolding of the cortex. In 

 European waters L. rosea, which has a diploid chromosome number 

 of twenty, is only known to produce tetraspores which apparently 

 arise without undergoing meiosis. Individual spores germinate to 

 give a new plant or else a whole tetrad may germinate to give a new 

 plant. In such plants of L. rosea, therefore, the gametophytic 

 generation is wholly suppressed and we have a diplont which be- 

 haves as a haplobiont in respect of its life cycle. In Pacific waters, 

 on the other hand, the records suggest that the species behaves 

 normally, whilst the other common species, L. clavellosa, also 

 behaves in the normal fashion. 



* Ceramiales 



The range of habit in this order is considerable even though the 

 basic construction is uniaxial throughout. There are simple, 

 branched filaments (e.g. Callithamnion, Antithamnion); leafy ex- 

 pansions (e.g. Delesseria, Laingta); nets (e.g. Claudea, Martensia: in 

 Claudea the net form originates by branching, in Martensia by 

 separation of cell rows); cartilaginous forms {Laurencia); poly- 

 siphonous and corticated polysiphonous forms {Polysiphoniuy 



