Fig. 146 Plumaria elegans. A, plant ( x f). B, antheridial ramuli 



( X 180). C, paraspores ( x 213). D, tetrasporic ramuli ( x 126). (A, 



original; B, after Drew; C, D, after Suneson.) 



The antheridia are borne on special branches, whilst the four- 

 celled carpogonial branch develops from the subterminal cell of an 

 ordinary branch. In northern waters P. elegans never bears sex 

 organs and only plants with paraspores are to be found, whilst in 

 southern waters the sexual (n = 3i) and tetrasporic plants (n = 62) 

 are predominant. Recent investigation has shown that in this species 

 we are concerned with a triploid race (n= 93) in the northern waters 

 which reproduces by means of paraspores. There is apparently no 

 relation between the triploid plants and the other two races, and, 

 furthermore, the triploid has the wider distribution because it is 

 able to penetrate into the colder waters of the north. Tetraspores 

 are to be found on the triploid plants but their chromosomal com- 

 plement and fate are not known. Although both tetra- and para- 

 sporangia arise from a single cell it is doubtful if the two structures 

 are homologous. The reasons for this are first, the difference in 

 chromosomal complement, secondly, the absence of any apparent 

 relationship with the haploid and diploid plants, and thirdly, differ- 

 ences in the mode of development of the para- and tetrasporangia. 

 This is the first cytological record of triploid plants in the algae. 

 Plumaria elegans is therefore tetraphasic morphologically and 

 triphasic cytologically. It is possible that other algae may exhibit 

 the same phenomenon. 



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