RHODYMENIALES 243 



separates in order to form the outer cell layers, although a few 

 longitudinal filaments are left in the centre. The plant, which is 

 enclosed in a thick gelatinous cuticle, may bear unicellular hairs 

 that have arisen from the epidermal layer. The adult thallus has 

 developed from a group of eight to twelve apical cells, each of 

 w^hich produces a longitudinal filament, whilst the 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 preliminary 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. De- 

 pending 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 fertilization one of these receives a process from the carpo- 

 gonium which carries with it the diploid nucleus. This auxiliary 

 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 L. rosea, therefore, the gametophytic generation is 

 wholly suppressed and we have a diplont which behaves 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. 



16-2 



