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 Polysiphonia, 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? Far-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 



