238 



RHODOPHYCEAE 



other two races, and, furthermore, the triploid has the wider distri- 

 bution 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 complement and fate are not known. Although both 

 tetra- and parasporangia 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, differences in the mode of development of the para- and 

 tetrasporangia. This is the first cytological record of triploid plants 

 in the algae. Paraspores are also known in the related genus 

 Ceramium but their cytology, and hence homologies, are not known. 



GIGARTINALES 

 Choreocolacaceae : Harveyella (after G. Harvey). Fig. 158. 



This and the closely allied genus Holmsella are monotypic 

 genera each containing a holo-parasitic species, whilst Choreocolax 



Fig. 158. Harveyella and Holmsella. A-E, stages in development of gonimo- 

 blasts after fertilization in Harveyella mirabilis. a = auxiliary cell, 5 = sterile 

 filaments. F, filaments of parasite, Holmsella pachyderma, in host. G, antheridia 

 of Harveyella mirabilis. H, tetraspores in Holmsella pachyderma. e = tracks left 

 after tetraspores have escaped. ^ = sterile cells, i = tetraspores in various stages, 

 ie = escaping tetraspores. (After Sturch.) 



is another parasitic genus very nearly related to them. Harveyella 

 mirabilis is parasitic on species of Rhodomela whilst Holmsella 

 pachyderma parasitises Gracilaria confervoides. They have little or no 



