136 Agnes Robertson. 
higher plants there is remarkable uniformity in the structure and 
method of division of the nucleus, and the number of chromosomes 
seems generally to be constant for any given species. It seems 
natural to expect that this number would be the same in closely 
related plants, and this turns out to be the case in a good many 
instances. Twelve seems to be the prevailing number in the 
gametophytes of the Coniferae (excluding Taxeae), sixteen is the 
reduced number in six species of Orchidaceae , 1 all belonging to 
different genera, which were examined by Strasburger and 
Guignard, while ten were counted by Strasburger and Frye in two 
species of Asclepias, 1 six by Coulter and Chamberlain, and Atkinson 
in Trillium recurvation 1 and T. grandifiorum ' and eight by Murrill 
and Land in two species of Silphium. 1 When there is a difference 
between the chromosome numbers in two closely allied plants it is 
worthy of notice that not infrequently they are related to one 
another in the proportion of one to two. For instance there are 
eight chromosomes in the sporophyte of Pallavicinia• and sixteen 
in Pelliap twenty in one species of Drosera 4 and forty in another, 
eight in the gametophyte of Pontederia 5 and sixteen in Eichhornia .* 
There is also a possible case of this particular variation occurring 
within the limits of the same species, for it has been recorded that 
there are twelve chromosomes in certain filaments of Spirogyra 
triformis , 4 whilst in others, indistinguishable from these in every other 
respect, only six are to be found. It certainly seems more likely 
that this is not a case of true dimorphism, but that the threads with 
six chromosomes had gone through a process of reduction in pre¬ 
paration for conjugation. However van. Wisselingh, who described 
the occurrence of these two forms, left this point undetermined. 
We have only referred at present to the structure of vegetative 
cells, but it is in the cytology of reproduction that clues to 
phylogeny are more readily to be found. Darwin in a classical 
passage quotes Owen who says, “ The generative organs, being 
those which are most remotely related to the habits and food of an 
animal, I have always regarded as affording very clear indications 
1 Coulter and Chamberlain. “ Morphology of Angiosperms,” 
1903, p. 81 and 82. 
2 J. B. Farmer. “ On Pallavicinia decipiens.” Annals of Botany. 
Vol. VIII. 1894, p. 35. 
3 J. B. Farmer. “ On Spore-Formation and Nuclear Division in 
the Hepaticae.” Annals of Botany. Vol. IX. 1895, p. 469. 
4 Rosenberg quoted by R. P. Gregory. “ Spore Formation in 
Leptosporangiate Ferns.” Annals of Botany. Vol. XVIII. 
1904, p. 445. 
‘ Coulter and Chamberlain. “ Morphology of Angiosperms.” 
1903, p. 81. 
8 C. van Wisselingh. 11 Ueber Kerntheilung bei Spirogyra.” 
Flora 1900. 
