L.T.4' 



P.A. 



C.H.\ 



disc 



^ ^M. Levillei 



/ X X ■■ X 



f 



CORTEX- 



=» B' (microscopic) //^ 

 2xoy 



4n^^ /? ^ Sporophyte 



(macroscopic) 



DISC 



Fig. 8 1 Mesogloia vermiculata. A, plant (sporophyte). B, apex of 

 filament with branches and beginning of cortication ( x 135). C, 

 unilocular sporangia. D, plurilocular gametangia on gametophyte. 

 E, diagram to illustrate construction of thallus (cental thread type). 

 C.H.= colourless hair, L.T.= leading thread with mtercalaiy 

 growth zone, P.A. = primary assimilator, S.A. = secondary assmii- 

 lator, 5.c.= secondary cortex, 5.5c. = secondary sub-cortex. F, 

 diagram to illustrate Ufe cycle. (A, C, D, after Tilden; B, E. F, 



after Parke.) 



unilocular sporangia during zoospore formation, and culture ex- 

 periments 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 ectocar- 

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

 develops into the characteristic basal disc from which the central 



150 



