482 BOTANY OF THE LIVING PLANT 



tetrad-division, then the cell of Rhopalodia would be diploid throughout 

 its vegetative divisions ; but that of Closterium would be haploid. On the 

 chromosome-definition such Diatoms would represent the sporophyte-genera- 

 tion, but the very similar Desmids the gametophyte (p. 401). 



In larger Algae the position of the event of reduction in the life-cycle is 

 se^n to vary even in organisms that are related to one another. Thus among 

 the Brown Algae, Dictyota has distinct though similarly formed and alternat- 

 ing sexual and tetrasporic plants, the former with 1 6, the latter with 32 chro- 

 mosomes. Here reduction accompanies the formation of tetraspores (p. 387). 

 But in Fucus there are no tetraspores, and reduction takes place before fertilisa- 

 tion, in the first divisions respectively of the oogonium and antheridium. The 

 doubling of them in fertilisation results on germination directly in the Fucus- 

 plant, which is accordingly diploid. There is here no alternation of dis- 

 tinct diploid and haploid generations. In the Red Seaweeds the converse is 

 the case. The relatively simple Nemalionales have been shown to carry out 

 reduction immediately after fertilisation. Here again there is no alternation 

 of generations, and the plant as such is haploid. But most of the Red Sea- 

 weeds have alternating generations, the one haploid, bearing gametes (gameto 

 phyte), the other diploid, bearing tetraspores (sporophyte) (p. 389). Such 

 a life-history corresponds to what is seen in Dictyota. But when we compare 

 the Nemalionales with Fucus we see that the former are haploid, the latter 

 diploid. The difference between them is the same as that already noted 

 between Closterium and Rhopalodia. It appears then that Algae may be (i) 

 haploid without any distinct alternation (Closterium, Nemalionales) ; (ii) 

 diploid without any distinct alternation (Rhopalodia, Fucus) ; or (iii) with 

 distinct alternation of generations (Dictyota, Polysiphonia, and other Red 

 Seaweeds). In. all of these there is only one constant feature, the alternation 

 of syngamy aria reduction. Those events may follow in quick succession, or 

 they may be separated by intervening somatic developments. 



It has been suggested as probable that the simpler condition, with only one 

 generation, as seen for instance in the Nemalionales, is more primitive than 

 that with two, as in Polysiphonia. In explanation how the latter may have 

 come itito existence two alternatives appear possible. Either an interpola- 

 tion of a sporophyte by successive steps, or a sudden deferring of reduction. 

 In the latter case it is possible to imagine a carrying over of the reducing act 

 to a later phase of the individual life. It has been suggested that the uni- 

 cellular spores (monospores) already known in the Nemalionales, may have 

 been in other Red Seaweeds converted into tetraspores by some sudden 

 deferring of the act of reduction. A final judgment on this question must 

 be held over till the necessary comparisons with allied forms shall have been 

 made. But meanwhile the fact that in certain Ferns the stage externally 

 that of the "gametophyte" may be diploid (Athyrium filix-foemina, var. 

 clarissima), and the stage externally that of a sporophyte haploid (Lastraea 

 pseudo-mas, var. cristata), would seem to indicate the possibility of the sugges- 

 tion being true (p. 350). 



The whole series of facts of alternation in the Thallophytes suggests that 

 the relative uniformity of the conditions, under which these successive phases 

 grow, has operated so as to stamp a high degree of uniformity upon the 

 somatic developments included in their life-cycle. The only constant rule 





