c3- 



1 AUTUMN 



<'—Q.. 



D 



protonema 

 WINTER 



dioecious adult \ 

 / I 



SPRING 



SUMMER 



Fig. 94 Life cycle of Colpomenia sinuosa. RD = probable place of 

 reduction division in life cycle. (Modified from Kunieda and Suto.) 



In A, hullosus the zooids from the plurilocular sporangia on the 

 macroscopic thallus do not fuse but germinate directly to give rise 

 to a series of plethysmothalli bearing plurilocular sporangia : these 

 tide over the winter season, and then in spring young Asperococcus 

 plants develop in place of the sporangia on the ectocarpoid plant- 

 ules. In the unilocular sporangia meiosis takes place and the zooids 

 develop into minute gametophytic plants that produce plurilocular 

 sporangia. If the gametes from these sporangia fuse the zygote 

 develops into a plantule from which a new macroscopic plant 

 arises, but if there is no fusion then they merely develop into a new 

 gametophytic generation. Sauvageau also reported that the zooids 

 from the unilocular sporangia may give rise to creeping filaments 

 which later produce young plantules of Asperococcus. This direct 

 reproduction of the macroscopic plants can only be explained by a 

 premature abnormal fusion of some of the zooids from the uni- 

 locular sporangia. It will be evident that direct alternation of 

 generations is obscured in this type through the number of possible 

 independent circuits and 'short-cuts'. Recently the hfe cycle in 

 Colpomenia sinuosa^ a member of a closely aUied genus, has been 

 described in detail (cf. Fig. 94). The adult plants are like Leathesia 

 in appearance, although they are essentially parenchymatous in 

 structure. It will be seen, however, that there are two morpho- 

 logically similar generations, the dioecious gametophj^es appearing 

 in spring and reproducing by means of anisogametes that are 

 formed in dissimilar gametangia. The zygote gives rise to new adult 

 asexual plants that reproduce by means of plurilocular sporangia 

 in autumn. This life cycle is somewhat different from the others 



167 



