Studies in the Dictyotaceae. 
I. The Cytology of the Tetrasporangium and the Germinating 
Tetraspore. 
BY 
J. LLOYD WILLIAMS, 
Assistant Lecturer in Botany , University College , Bangor. 
With Plates IX and X. 
I N 1897 I published an account of the discovery of motile antherozoids 
in Dictyota 1 . At the Bristol meeting of the British Association (1898) 
the mode of fertilization was described. The cytology of the antheridia 
and oogonia and of the apical region of the tetraspore plant was also ex- 
plained and illustrated by means of figures. It was found that the full 
number of chromosomes obtained in the apical nuclei of the asexual 
plants, while in the single division of the oogonium and in the early 
divisions of the antheridium the reduced number prevailed. This necessi- 
tated a more careful search for the actual reduction division. Ultimately 
it was found to be the first division in the tetraspore mother-cell. Mean- 
time Mottier 2 in 1898 announced his discovery of centrosomes in Dictyota. 
Subsequently (in 1900 3 ), he published a detailed description of the reduction 
division, as well as of the second division of the tetraspore mother-cell and 
of the vegetative mitoses in the thallus-cells. I have worked out the 
cytology of all the various kinds of cells in the three forms of Dictyota , 
male, female and asexual, and a considerable body of facts has been 
gathered relating to the natural history of the living plant, both under culti- 
vation and in its natural habitat. As far as cytological evidence is concerned 
there seems to be no reason to doubt that we have here a clear case of 
alternation of generations. Publication of the results has hitherto been 
delayed in the hope of completing the evidence by actual cultivation of 
the plant from spore to spore. All the numerous attempts made to produce 
this result have hitherto failed, owing to the difficulty of getting the plant 
to fruit in cultivation, and it has been thought best not to further delay 
1 Williams, Annals of Botany, xi, 1897. 
2 Mottier, Ber. d. deutsch. Bot. Gesellsch., xvi, 1898. 
9 Mottier, Ann. of Bot., xiv, 1900. All subsequent references to Mottier will be to this paper. 
[Annals of Botany, Vol. XVIII. No. LXIX, January, 1904.] 
