234 
F. E. Fritsch. 
favour essentially the homologous view of alternation, and ob¬ 
viously opens up many possibilities for experimental research into 
this vexed question. 
The writer has no intention of discussing the relative merits 
of these different theories. The purpose of the present communi¬ 
cation is, in the first place, to sift the ground for the characters of 
the ancestral group from which the higher plants may have arisen. 
No one is probably prepared to dispute that the latter have a 
common origin, and that their ancestors were of the type of the 
present-day Thallophyta. Since all the groups above the level of 
the latter have pure green chloroplasts, with starch as a customary 
product of assimilation, it is to be presumed that their ancestors 
exhibited these features. Moreover, the spermatozoids appear 
invariably to be isokontan. This leads us inevitably to look for the 
ancestry of the higher forms among the Isokontae . 1 The two gene¬ 
rations in the higher groups are in general characterised by certain 
prominent features, in considering which we may leave the highly 
specialised Phanerogams out of the realm of our discussion. 
The gametophyte is evidently typically prostrate and dorsi- 
ventral, the assumption of a radial construction being rare, and a 
mark of specialisation. The sexual organs seem primitively to have 
been borne on the upper surface, as we see it at the present day in 
most of the Hepaticse, although tending to shift to the lower side, 
where more ample protection is obtainable. In practically all the 
Pteridophyta and all those Bryophyta regarded as the more 
primitive, the gametophyte retains a thalloid differentiation. 
The sporophyte, on the other hand, is typically upright, and 
radial in organisation. This is quite patent in the Bryophyta, and 
it appears to be generally accepted that the sporophyte, in the 
Pteridophyta, was primitively radial . 2 A second feature of the 
sporophyte, in the Archegoniatae, is the tendency to differentiate a 
main axis, with lateral appendages subservient to assimilation and 
the production of the asexual cells. 
The two generations of Archegoniatae may, therefore, be briefly 
characterised as follows:—The gametophyte a dorsiventral prostrate 
thallus, with the reproductive organs primitively on the upper 
surface; the sporophyte an upright structure, with radial organisa¬ 
tion and a tendency towards peripheral placing of the assimilatory 
1 For a discussion of Schenck’s hypothesis as to the origin of the Arche- 
goniates from the Phieophyceae, see p. 13. 
2 cf. Bower, The Origin of a Land Flora, 1908, pp. 363 and 625. 
