241 
Algal Ancestry of the Higher Plants. 
affords indications of another possible mode of origin for the two 
alternating generations, an antithetic one, that seems most applicable 
to the case of the Bryophyta. This concerns the much-discussed 
Coleochcete. Here alone among the Isokontaedo we get any pronounced 
indication of an intercalated phase, due to the elaboration of the 
zygote, although minor instances are afforded by several other 
filamentous members (e.g. Ulothrix, Oedogonium). Comparisons 
between Coleochcete and Riccia, the Liverwort with the simplest 
type of sporophyte known, have been instituted by many Botanists, 
and it is unnecessary to enter into details here. 
The value of Coleochcete, in relation to the origin of the two 
alternating generations in the higher plants, has been called into 
question by many in recent years, because the cytological features 
are not in accord with those obtaining in the latter. Allen 1 has 
shown that reduction in chromosome-number takes place on the 
germination of the oospore, so that the 16-32-celled plantlet arising 
from the latter corresponds cytologically with the ordinary sexual- 
organ-bearing thallus, and not with the diploid sporophyte of the 
higher plants. Should this, however, be taken as in any way 
invalidating the value of Coleochcete as an instance of the inter¬ 
calation of a new generation in the life-cycle by the elaboration of 
the zygote? In the group of the Algae everything is still in a 
condition of fluctuation, and we know that reduction in chromo¬ 
some-number occurs at varying points in the life-cycle, sometimes 
on gametogenesis, sometimes after sexual fusion. 2 Thus reduction 
rendered necessary by the occurrence of sexuality will have taken 
place at different stages and will only gradually have become fixed 
at the stage of sporogenesis. Perhaps it was only with the estab¬ 
lishment of the tetrad-division, so typical for the production of 
spores in the higher plants (as well as in Dictyota and many 
Rhodophyceae), that reduction became located at a definite point 
in the life-cycle. 
In considering the possible origin of the sporophyte in Bryo¬ 
phyta by gradual elaboration of the zygote, as we see it indicated 
in Coleochcete, it is not even necessary to ignore the essential 
construction of the thallus in the Chaetophorales as a whole. The 
two main groups of the Bryophyta appear to have diverged at a 
\ 
1 Ber. deutsch. Bot. Ges., XXIII., 1905, p, 285. 
2 Redaction takes place on gametogenesis, for instance, in Fucaceae. To 
regard the thallus in this group as being a sporophyte, on this basis, is surely 
the height of absurdity! cf. also Farmer, in New Phytologist, VIII., 1909, 
pp. 112-114 ; and Tansley, in New Phytologist, XI., 1912, p. 210. 
