Thalassiophyta and Algal Ancestry of Higher Plants 175 
such a thallus developed in two directions (as indeed present-day 
Trentepohlias illustrate) and that as an adaptation to subaerial 
conditions relegation of the sexual reproductive organs to the 
creeping base took place. Such a theory is in harmony with the 
prevalent dorsiventrality of the gametophyte and helps to explain 
the persistence of “aquatic” fertilisation in Bryophyta and Pterido- 
phyta 1 . Whether alternation was already established or whether 
it became subsequently established is immaterial; as soon as two 
phases were evolved they would be bound gradually to diverge, the 
one (gametophyte) losing the erect system (except in Mosses?), the 
other (sporophyte) the prostrate system. In the dual development 
of the primitive type of thallus we have all the potentialities for the 
evolution of the two highly contrasting phases in the life-cycle of 
land-plants. On this view they were probably ab initio identical. 
Let us consider the position of such a filamentous form, as it 
gradually became exposed by the rising of the land out of the water. 
Assuming the power of adaptation to terrestrial existence so strongly 
evidenced by the Green Algae of the present day and the general 
condensation of the exposed parts above postulated, we should have 
a dense tufted erect growth (probably of no appreciable height) 
arising from a creeping basal system which would probably not be 
as much affected at first at least 2 . Such a form would not be 
subjected to the vicissitudes to which larger forms would be exposed 
and, with the more favourable conditions for photo-synthesis and 
the better oxygen-supply, might well make relatively rapid headway. 
It is difficult to conjecture what direction the advance might take 
at first, but there may well have been an early departure from the 
filamentous condition and the adoption of a more massive habit, 
either in both parts of the thallus or only in the creeping base. It 
may be an open question whether the transmigrant Alga had already 
attained to oogamy. There are no oogamous Green Algae in the sea 
(cf. Oltmanns, 11. p. 176), so that oogamy may either have arisen 
subsequent to transmigration or the oogamous forms were the 
successful transmigrants 3 . At whatever stage oogamy may have 
been evolved fertilisation in situ and post-sexual nutrition would be 
a likely consequence of the increased efficiency of the organism due 
to the acquisition of a more massive body. It is to be noted that 
among Seaweeds the features in question are exemplified only in the 
1 Regarding Mosses, see Fritsch, p. 250. 
2 Since probably still periodically inundated. 
3 In the latter case it is however again difficult to conceive that no oogamous 
forms should have been left behind. 
