EVOLUTION 259 



it would be found that they are really homosporous, in much the 

 same way as Thomson has suggested that the so-called micro- 

 and megaspores of the angiosperms are really homosporous. If 

 this were found to be true, then the problem of the origin of 

 heterospory in the Fucales would be disposed of and the evolu- 

 tionary problem much simplified. It is also possible that the 

 explanation of heterospory and heterangy in the Fucales is to be 

 found in the retention on the parent thallus of heterothallic game- 

 tophytes, the stimulus provided by their presence being responsible 

 for the modification of the original morphologically identical 

 unilocular sporangia. 



One or two authors have recently suggested that an origin for the 

 Fucales should be sought for among the Mesogloiaceae and 

 Encoeliaceae (Colpomenia), but the evidence produced cannot be 

 regarded as wholly convincing. The most recent account by Delf 

 (1939) considers this problem in some detail. In adult plants of 

 Fucus there is an apical growing cell which is now known to arise as 

 follows. In the sporeling a group of apical hairs is formed at the 

 growing point, each hair possessing basal (trichothallic) growth as 

 in the Ectocarpales. These hairs die off and the lowest cell of one 

 hair gives rise to the four-sided apical cell of the adult thallus. 

 A similar behaviour of the apical hairs is to be seen in Acrothrix 

 (Mesogloiaceae). New growth from wounded tissue in Fucus also 

 develops a new apical cell from such trichothallic hairs, whilst the 

 development of the cryptostomata and conceptacles also appears to 

 be analogous. It is further suggested that in gross structure, e.g. 

 primary and secondary medullary filaments and the assimilatory 

 tissue, the thallus of Fucus shows considerable resemblance to that 

 of Eudesme as illustrated in fig. 95. Difficulties associated with this 

 interpretation must be concerned with the differences in size of 

 the thalli and also the presence of heterospory and heterangy. The 

 gametophytes of the Mesogloiaceae reproduce by means of pluri- 

 locular sporangia which do not exhibit either anisogamy or heter- 

 angy nor do the sporophytic plants exhibit either heterospory or 

 heterangy. Recent work, however, has shown that the gametophyte 

 of Colpomenia sinuosa bears organs that must be regarded as 

 relatively simple antheridia and oogonia, so that there is here an 

 example of heterangy associated with anisogamy (cf. p. 154). 



Whether the Fucales are derived from the Laminariales or 



17-2 



