AMPHIBIOUS HABIT 81 
cytological investigation progresses, but those quoted will suffice to show that, 
for those who accept the cytological distinction of the alternating generations, 
the mere distinction on ground of form is insufficient. 
The existence of certain cases of alternation, demonstrated cytologically, 
where, as in Dictyota, the form of the two generations is substantially the 
‘same, and of other cases in which, as in the Archegoniatae, the genera- 
tions differ fundamentally in form as well as in points of structure, raises 
a physiological question which has wide bearings. The external conditions 
to which, in the past as well as now, the two generations have been or are 
‘exposed must be considered in their relation to such differences. Now, in 
Dictyota, and also in Polysifhonia, since ‘the two forms grow on the same 
‘coasts, at the same time, and about the same spots, it is obvious that the 
‘conditions to which they are exposed are substantially the same: knowing 
-how closely form is dependent upon circumstance it is not surprising that 
‘the two generations should be so much alike. 
In the case of the Uredineae, again, the external circumstances of 
the two cytologically distinct stages of the life are alike, both being 
internally parasitic: in form their mycelial stages are also alike, and their 
difference of character only becomes apparent on the formation of the 
propagative organs. Similar remarks will apply, perhaps with less cogency, 
to the post-sexual developments in certain Ascomycetes, the circumstances 
of which do not differ materially from those of the pre-sexual phase, and 
in this may be seen some illumination of the fact that they show for the 
most part mere variants upon the simple filamentous form. Thus, for 
‘certain Thallophytes in which cytologically distinct generations have been 
recognised, there is a remarkable similarity in form of the two generations: 
this probably finds its true explanation in the fact that the biological 
‘conditions to which they are respectively exposed are relatively uniform, 
and have been so throughout their descent. 
But with the Archegoniatae the case is quite different: the cyto- 
logically distinct and alternating generations differ widely in their characters 
of form and structure. The question, therefore, presents itself, what is 
the biological and ultimately the phylogenetic bearing of this very obvious, 
and at the same time widespread fact? FFar-reaching it is indeed, for 
it affects all the characteristic vegetation of the land. Taking, first, as 
the most prominent example, a typical Fern, the gametophyte, or 
prothallus, shows in its more delicate structure and in its habit, as well 
as in the way in which the sexual process is effected, an adaptation to 
moist conditions, under which it grows best; while its ultimate function, 
that of sexual reproduction, cannot be carried out without the presence 
of external fluid water: it is, in fact, typically semi-aquatic in its nature, 
sharing many of its main characters with the green Algae from which 
we have some reason to believe that the land-flora originated. The 
sporophyte, on the other hand, is fitted by its more robust texture, as 
well as by its differentiation of tissues, for successfully enduring exposure 
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