ECTOCARPALES 141 



growth they may appear to be terminal. The unilocular sporangia 

 are ovoid and are borne at the base of the cortical filaments, but the 

 elongate plurilocular sporangia, which incidentally are only known 

 for M. Levillei, replace the terminal portion of the assimilatory 

 hairs and hence are always stalked. Meiosis takes place in the 

 unilocular sporangia during zoospore formation, and culture 

 experiments on M. vermiculata carried out by Parke (1933) have 

 demonstrated conclusively that the adult macroscopic plant of 

 summer and autumn is diploid, the zooids from the unilocular 

 sporangia germinating into a minute winter gametophyte (haploid 

 adelophycee form) that bears plurilocular sporangia of an ecto- 

 carpoid type. The zooids from these sporangia fuse and the zygote 

 develops into the characteristic basal disk from which the central 

 erect filament of the macroscopic plant arises. There is thus 

 an alternation of morphologically distinct generations in this 

 species. The fate of the zooids from the plurilocular sporangia 

 of M. Levillei is not known. 



Mesogloiaceae : Eudesme (well-binding). Fig. 95. 



E. virescens, which is the type species of this genus, has recently 

 been removed from the genus Castagnea to which it is very closely 

 allied in structure. The branched mucilaginous plants differ from 

 Mesogloia fundamentally in the presence of more than one central 

 strand in the medulla. The primary filaments in the medulla, which 

 originate from a basal disk, have an intercalary growing zone and 

 terminate in a colourless hair, and as branching takes place from 

 these primary filaments laterals may develop in such a manner as to 

 make it difficult to distinguish them from the primaries. The cortex 

 is composed of club-shaped primary and secondary assimilatory 

 hairs arranged either singly or in falcate tufts. The unilocular 

 sporangia develop as outgrowths from the basal cells of the primary 

 assimilatory filaments, whilst the plurilocular sporangia appear in 

 secund rows on the outermost cells of the same type of filament. 

 The zooids from the unilocular sporangia germinate immediately, 

 or else some considerable time may elapse, perhaps as much as 

 3 years according to some observers, before any development takes 

 place. They give rise to a microscopic plethysmothallus on which 

 plurilocular gametangia similar to those of Mesogloia are to be 

 found. After zooids have been liberated from the plurilocular 



