REPRODUCTION 247 



antheridia and oogonia as modified plurilocular sporangia. This 

 concept is inevitably bound up with the phylogeny of the Phaeo- 

 phyceae because one can either read them as a series commencing 

 with the undifferentiated plurilocular gametangia of the Ecto- 

 carpales, or else one can regard these structures as reduced anther- 

 idia and oogonia in which differentiation has been completely lost. 

 (zb) The presence of a unilocular sporangium always indicates 

 the presence of a diploid thallus, and it invariably gives rise to 

 haploid zooids. 



Table II 



I. One kind of plurilocular sporangium. 



(i) Uni- and plurilocular sporangia on the same individuals, e.g. Ectocarpus 



(2) Uni- and plurilocular sporangia on different individuals, e.g. Sphacelaria 

 hipinnata, Cladostephus. 



II. Two kinds of plurilocular sporangia. 



(i) Meio- and megasporangia, e.g. E. virescens. 



(2) Antheridia and female gametangia ( = plurilocular sporangia). 



{a) Unilocular sporangia on separate plants, e.g. Sphacelaria hystrix, 

 Halopteris filicina. 



(b) Unilocular and both gametangia all on separate plants, e.g. Sphace- 

 laria Harvey ana. 



(3) Antheridia and oogonia ( = plurilocular sporangia). 

 Unilocular sporangia on separate plants, e.g. Dictyota, Laminaria. 



(4) Antheridia and oogonia representing modified micro- and megaspor- 

 angia ( = plurilocular sporangia). 



Unilocular sporangia on same plant, e.g. Fucales. 



{zc) A haploid zooid, irrespective of the nature of the structure 

 in which it was produced, can behave either as a gamete or as an 

 asexual zooid. 



(3) In many of the types it cannot be said that there is a regular 

 alternation of cytological or morphological generations, even 

 though it is potentially possible. Although by no means entirely 

 satisfactory, in a good many cases the race cycle can perhaps be 

 best described as possessing an irregular alternation of generations. 



(4) Theoretically it is obvious that there are three possibilities 

 which can be suggested in order to explain the origin and develop- 

 ment of the Phaeophyceae : 



A. Plants that are haploid throughout their Hfe cycle, except for 

 the zygote, represent the primitive condition, and the diploid stage 



