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.!_ 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 fr6m 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 Dzctyota 
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 Cwufleria: they are known as the Cutlerta and 
Aglaozonia stages.?. This case is quoted here as showing that in plants 
probably akin to Dictyota, the exactitude of the alternation in not maintained. 
But this fact comes out much more strongly in the case of Fucus, in 
1]. Lloyd Williams, ‘‘Studies in the Dictyotaceae,” Aznads of Botany, 1904; TD. M. 
Mottier, ‘‘ Nuclear and Cell Division in Dzctyota dichotoma,” Annals of Botany, 1900. 
2See Oltmanns, Worph. u. Biologie der Algen, Jeua, 1904, p. 396, etc., where the 
current literature is fully dealt with. 
