39 o BOTANY OF THE LIVING PLANT 



Polysiphonia, where tetraspores and sexual organs are borne on separate 

 plants. The carpospore is diploid and on germination gives rise to a tetra- 

 sporic plant (sporophyte) which bears tetraspores, reduction taking place 

 in their formation. The haploid tetraspore grows into the sexual plant. 

 The fusion of a spermatium and a carpogonium produces the diploid carpo- 

 spore : and so on. There is thus a regular alternation of tetrasporic and 

 sexual plants But in form these are closely alike. The case is comparable 

 with that of Dictyota ; and questions of the origin of alternation in the Red 

 Algae are raised similar to those for the Brown. 



Alternation of Generations in the Algae. 



The very considerable diversity of life-cycle shown by the Green, Brown and 

 Red Algae has been made evident in the preceding pages. At least four main 

 types can be distinguished. Thus most of the Chlorophyceae and Red Algae 

 such as Nemalion are haploid organisms, with reduction taking place at the 

 first division in the zygote. In these organisms the diploid condition is confined 

 to the zygote itself. In a second type of life-cycle, typical of some of the 

 Siphonales probably of all the Bacillariophyceae (Diatoms), and of the Fucales 

 among the Brown Algae, the plant is diploid ; reduction division takes place 

 at the formation of the gametes, and the haploid condition is limited to the 

 brief phase prior to the fusion of the gametes. A third type consists of the 

 alternation of two morphologically similar individuals, one haploid and gameto- 

 phytic, the other diploid and sporophytic. Such isomorphic or homologous 

 alternation is found in the Cladophorales and Ulvaceae among Green Algae, in 

 Ectocarpales and Dictyotale< among Brown Algae, and is typical of all the more 

 advanced Orders of Red Algae ; in the Brown and Red Algae reduction division 

 takes place in the unilocular sporangia and tetrasporangia respectively. 

 Lastly, heteromorphic alternation is met with in such Brown Algae as the 

 Cutleriales and Laminariales , and it may be, if we accept one interpretation 

 given above, also in the Fucales. In these instances the very large and con- 

 spicuous sporophytic plant stands in marked contrast to the minute gameto- 

 phyte, but both are equally required to complete the cycle. Such Algae not 

 only remind us strongly of the life-cycle in the Pteridophyta, but might be 

 regarded as another instance of the rise and predominance of the diploid 

 generation. But from a knowledge of the other types of life-cycle given 

 above it will be seen that while the Hofmeisterian doctrine of alternation — 

 founded originally for higher plants — applies in detail to some Algae, it cannot 

 be extended to the group as a whole. (See Chapter XXXIV.) 



Comparison with the Bryophyta, Pteridophyta, and Seed-Plants suggests 

 that it is in the tetraspores of the Algae that the nearest correlatives are found 

 to the spores produced in them by tetrad-division. In so far as their place 

 in the chromosome-cycle is concerned these spores may be held as comparable 

 with those of Archegoniate Plants. In Chapter XXXIV. the general relation 

 of the somatic to the chromosome-cycle is considered at some length. The 

 view is there advanced that the latter is the more stable in plants at large, and 

 is therefore to be held as the more important in comparison, with its normally 

 alternating events of syngamy and reduction. 



