2 43 
Algal Ancestry of the Higher Plants. 
It must be confessed that, in this series, almost every conceiv¬ 
able type of growth-form and habitat has been realised. It does 
not, therefore, appear a forced assumption to seek in a group with 
such unlimited potentialities those evolutionary tendencies which 
gave rise to the higher plants. 
In discussing the alternation of generations of the Archegoniatae, 
much use has been made of representatives belonging to other 
groups of the Algae, in particular of the Phaeophyceae. The latter 
show many interesting parallels with the Isokontae and afford very 
useful data in connection with the probable mode of origin of the 
higher plants, but in utilising the information derived from this 
group it is necessary to maintain the proper perspective and to 
realise that the Brown Algae cannot be regarded as anything else 
than a side-line of evolution (cf. below). In many respects, morpho¬ 
logical and anatomical complexity of the thallus, differentiation of 
the sexual organs, and development of well-marked alternation, 
they have gone much further than the Isokontae, and it is just in 
the evidence to be derived from these more advanced characteristics 
that the value of the Brown Algae lies. We may suppose that the 
more advanced types among the Isokontae were lost during the 
vicissitudes that must necessarily have accompanied establishment 
on dry land, whilst the Phaeophyceae, which have remained a 
marine group, have preserved numerous traces of this more advanced 
development. It is certainly significant that the Isokontae show 
but little of the morphological elaboration seen in the Brown and 
Red Algae. 
At this point reference may be made to Schenck’s attempt to 
derive the Archegoniatae and the Characeae from a Phaeophyceous 
ancestry. 1 Whilst in no way underestimating the value of the 
detailed comparisons which he makes between vegetative and 
reproductive organs in the Brown Algae and the higher groups, the 
writer cannot go with him to the extent of actually deriving the 
latter from this group of Algae. The Phaeophyceae as a whole are 
distinguished from the Isokontae and the higher plants alike by 
different pigmentation of the chloroplasts, different products of 
assimilation, and especially by a very characteristic type of motile 
element. In particular stress may be laid upon the motile element 
(whether zoospore or gamete) which is characterised by its 
laterally attached cilia, the one pointing forwards, the other back¬ 
wards, and the close relation between their position and that of the 
1 Engler’s Bot. Jahrb, XLII., 1909, pp. 1-37. 
