1920] on The Earliest Known Land Flora 145 



On the other hand, comparison of the Bryophytes will leave 

 little doubt that the sporangium of the -Psilophy tales and the sporo- 

 gonium are kindred structures. If this be so, then we shall see 

 linked together by comparison with these new fossils, not only the 

 sporogonia of Bryophytes and the sporangia of ferns, but even 

 the pollen-sacs and ovules of flowering plants. Long ago it was 

 remarked that the widest gap in the sequence of plants was that 

 between the Bryophytes and the Pteridophytes. It is within this 

 gap that the newly discovered fossils take their natural place, acting 

 as synthetic links, and drawing together more closely the whole 

 sequence of land-living, sporangium-bearing plants. We still await 

 with interest the considered comparisons of the authors of these 

 notable memoirs, though they have already pointed out several fertile 

 lines. But those who have been deeply engaged in comparative 

 morphology may be excused for stating how these new facts strike 

 them. Clearly the morphology of land-living plants is again in the 

 melting-pot. It will emerge strengthened by new and positive facts, 

 and refined by comparisons which can now be based upon solid data, 

 and less than before on mere surmise. 



The new facts are thus seen to link the Bryophytes and the 

 Pteridophytes more closely together than ever before. It may be 

 that these two great phyla of land-living plants have themselves 

 diverged from some common source still unknown. But that source 

 is reflected more nearly in the Lower Devonian plants than in any 

 other known forms. If that be so, whence may these still more 

 primitive plants have sprung ? The view has always been enter- 

 tained that the Algae preceded land-living plants. For long the 

 fresh-water green Algae were believed to have provided the source. 

 Latterly from the Continent, but notably also here at home, at the 

 instance of Laug and of Church, the belief has swung round towards 

 marine forms. Highly specialized Algae flourish on every rocky 

 shore. Some of these show alternation. All are rootless. .Some have 

 a differentiation of their branch-system which prefigures the relation 

 of leaf and axis. Not a few of the Red Seaweeds have spore-tetrads 

 borne internally, and located in the ends of specialized branches 

 called stichidia. These are not altogether unlike sporogonia, or the 

 large sporangia of the Lower Devonian plants. "We may well regard 

 it as improbable that any direct transition of such specialized types 

 to a land-habit took place, though this has been hinted at more than 

 ouce. But at least corresponding features of external differentiation 

 and of spore-production are present in both. Homoplasy may be the 

 real explanation of the likeness, but still the similarity exists. 



From what has been said it is clear that during the years of 

 war plant morphology has entered upon a new phase. The problems 

 of origin of root and axis and leaf and sporangium have been pro- 

 pounded afresh in terms of the new discoveries. The day is past of 

 that vague surmise on these points which has bulked so largely in 



Vol. XXIII. (Xo. 1U) l 



