plantule 

 2x 



cx 



r 



^9 



L 



^i- plethysmothallus 

 2X 



f 



B 



prothallus 

 X 



plantule 

 2x 



Fig. 93 Diagram of life cycles of Dictyosiphon foeniculaceus (A) 



and Asperococcus bullosus (B) 



enables the development of the macroscopic phase to take place 

 once more. There is no evidence of either meiosis or of gametic 

 fusion. In A.fistulosus it would appear that the life cycle is depend- 

 ent upon the behaviour of the zooids from the unilocular sporangia 

 where meiosis has been shown to take place. If they fuse, the 

 zygote develops first into a 'streblonema' phase, so-called from the 

 brown alga it resembles, and then into a plantule from which a new 

 adult plant can arise. In this case there is no evidence for the exist- 

 ence of a gametophytic generation, nor has any evidence been 

 obtained to show that such streblonemoid plants can reproduce 

 themselves by means of sporangia. If no fusion of the zooids from 

 the unilocular sporangia takes place the *streblonema' phase is again 

 produced parthenogenetically, but under these circumstances 

 plurilocular sporangia are formed which give rise to a new ^streblo- 

 nema' generation, nor has any investigator under such conditions 

 succeeded in obtaining macroscopic plants again and so it has been 

 suggested, therefore, that sex has been inhibited in these plants. 



i66 



