66 ALTERNATION IN THE THALLOPHYTES 



anticipate that one or other of these locations will be found to be a 

 general feature of those plants where there is no obligatory succession of 

 phases, and their somatic condition would accordingly': be in the former case 

 haploid, in the latter diploid. With this remark they must be left on one 

 side for the present. 



In other cases, however, a succession of obligatory phases, defined on 

 the one hand by the incident of sexual coalescence, and on the other by 

 reduction, has been brought to light ; in fact, examples of alternation 

 have been found among the Thallophytes, showing cytological limits closely 

 comparable with those which have been accepted in the previous chapter 

 for the alternating generations in the Archegoniatae. Among the Algae 

 one of the best cases of this, substantiated on both cultural and cytological 

 evidence, is that of Dictyota dichotoma, Lamour. It has long been known 

 that the tetraspores, antheridia, and oogonia of this plant are distributed 

 on different individuals, but it has only recently been shown in what 

 relation these plants stand to one another. 1 We now know that the number 

 of chromosomes in the somatic divisions of the plants which bear antheridia 

 and oogonia is 16 : that there is no change of chromosome-number in 

 the formation of the sexual cells, but that the fertilisation results in a 

 zygote which on germination gives a plant with 32 chromosomes in its 

 somatic divisions. This plant bears tetraspores ; but in their production 

 the mother-cell, on dividing its nucleus into two and then into four, 

 shows a reduction to the original 16, the details of the process being 

 closely comparable to those in the tetrad-reduction of Archegoniates and 

 Phanerogams. The tetraspores on germination give plants which show 16 

 chromosomes on their somatic divisions, and thus correspond to the original 

 sexual plants. The only gap which is left in the full demonstration of 

 the life-cycle, both by cultures and by cytological observation, is that the 

 plants raised by cultures from tetraspores have not yet been seen to bear 

 sexual organs : but still they correspond in their chromosome-number. 

 Here, then, is a succession of phases which appears to be obligatory, 

 involving two stages which have the same chromosome-relation as the 

 alternating generations in the Archegoniatae. But there is this difference : 

 that in external form and structure the two alternating generations of Dictyota 

 are substantially alike though the one is haploid and the other diploid. 



Somewhat similar phases, which alternate in a less exact and obligatory 

 manner, and in which the cytological details have not yet been observed, 

 are seen in the life of Cntlcria : they are known as the Cutlcria and 

 Agltio^onia stages. 2 This case is quoted here as showing that in plants 

 probably akin to J^ntyota, the exactitude of the alternation in not maintained. 

 But this facl comes out much more strongly in the case of Fitcus, in 



'I. J.loyil Williams, " Suidirs in il-c Diclyotaceae," Annals of /inlaity, 1904; D. M. 

 Mnttii-r, " Nuclear and Cell Division in Dictyota dichotomy," Annals of Botany, 1900. 



Ohmunns, ,]/<>////. //. / ,/,>- .-//</-,//, Jena, 1904, p. 396, etc., where the 



current literature i i lullv lralt will). 



