176 



KNOWLEDGE. 



June, 1915. 



groups in this respect, and, as we shall see, this 

 has an important bearing on the problem under 

 discussion. In the Spcrmaphyta the gametophytes 

 — more especially the female — arc dependent on 

 the sporophytc. Our problem is to show how these 

 groups are related to one another and to the 

 supposed primitive aquatic flora. 



As was indicated in the previous article, a series 

 of living forms cannot represent a true evolutionary 

 series, and evidence derived from such must be 

 used with caution. Hence attention is turned to 

 fossil forms which have thrown much light on the 

 relationships of living forms, especially in the 

 Pteridophyte. Fossil Algae are, however, often of 

 doubtful determination, and give few clues to 

 structure, so that on the subject of the first stages 

 of evolution from the Algal to land types the record 

 of the rocks is very imperfect, or, from the delicacy 

 of the forms, is perhaps non-existent. In such a 

 case we can only turn to biological reasoning 

 founded on existing types, using the principles of 

 progressive evolution. 



In some respects, the Bryophyta appear to be 

 more nearly allied to the water forms — the gameto- 

 phyte is independent and the conspicuous phase ; 

 differentiation of structure is comparatively slight, 

 and perhaps the majority of the members, of the 

 Uverworts especially, are moisture-loving in habit. 

 Suppose we regard this gametophyte as the direct 

 descendant of the thallus of an Alga bearing sex- 

 organs, what is the relation of the sporogonium ? 

 The most probable answer is perhaps as follows. 

 In the Green Algae generally the oospore either 

 germinates directly to produce a new thallus 

 (e.g., Vaucheria), or it may undergo one or two 

 divisions before doing so. In Oedogonium the ger- 

 minating oospore organises four zoospores (see Figure 

 157) — asexual spores — each of which produces a new 

 plant. The nucleus of the zygote of Spirogyra also 

 divides into four, but three of the nuclei disappear 

 (see Figure 158) — a kind of degenerate form of the 

 tetrad division seen in Oedogoniuni. In Oedogonium 

 it may be said that there is a short post-sexual or 

 sporophytic phase. Suppose that this tendency 

 is carried further, so that several successive divisions 

 take place in the oospore before it finally breaks up 

 into asexual reproductive units ; the structure 

 produced is essentially a simple sporophyte. 



In the simpler liverworts, such, for example, as 

 Riccia, the sporophyte is formed by the oospore 

 undergoing repeated division until a sporogonium is 

 produced, which consists of a wall enclosing a mass 

 of spores— just such a type as we have imagined 

 for our Algal type above (see Figures 159 and 162). 



Passing to members of the Bryophyta which 

 show a more complex structure for the sporogonium, 

 we can imagine these to be derived by the conversion 

 of originally fertile tissue into sterile, with a con- 

 sequent slight delay of spore production, but with 

 ^•Iie formation of a structure better adapted for 



the nourishing of the developing spores, and also, 

 finally — an important point — for their dispersal, 

 as they are no longer self-dispersed. In this way 

 a series showing increasing complexity of structure 

 may be constructed from the living Bryophyta. 

 Thus, without assuming any direct relationship 

 among the forms, we may contrast in this way 

 Riccia, Marchantia, Anthoccros of the liverworts, 

 and Sphagnion, Funaria of the mosses, the whole 

 forming an upward scries of the type described, 

 while there is little doubt that in evolution Funaria 

 represents a higher form than Riccia (see Figures 

 160, 161, and 163). 



Thus we outline the argument, which has been 

 fully stated by Bower, * that here the sporophyte 

 is to be regarded as an elaboration of the fertilised 

 egg cell, and is a new structure not really de- 

 veloped to any extent in the Algae. This is not, 

 however, the only theory of the sporophyte, as 

 will be seen below. 



There is no true evidence of any forms among 

 the Algae which really approach the Bryophyta. 

 Colcochaete was at one time thought to do so, but 

 this view cannot now be held. The theory rests, 

 therefore, very largely on the analysis of biological 

 principles and their application to this case. There 

 is, as has often been said, a big break in the chain 

 of evolutionary forms which connect the moss 

 plants with the Algae. 



The other groups now engage attention. Are 

 these descended in any way along the same line as 

 the Bryophyta or along a totally distinct one ? 

 It may be said at once that the botanical camp is 

 divided upon this question, the " factions " reading 

 the meagre evidence in different ways. Although 

 the Pteridophyta are well represented in the fossil 

 flora, in deciding this question the evidence again 

 seems of little use, for no really primitive fern plants 

 have as yet been identified. The fossil forms 

 greatly enlarge our idea of the group, but they do 

 not extend its boundaries till they overlap with a 

 lower one. 



The two principal theories of their origin are 

 briefly here set out. 



That which will be first taken assumes that the 

 Bryophyta and Pteridophyta are ultimately of the 

 same or closely related origin, and the sporophytes 

 are regarded as really of the same nature. The 

 differentiation of this in the Pteridophyta has 

 obviously gone very much further, but this is the 

 result of the same general processes. The 

 sterilisation has gone so far that a self-supporting 

 structure, with well-organised root and shoot 

 systems, has been produced, as, for example, in the 

 ferns, selaginellas, and so on. In consequence, the 

 proportion of spore-producing tissue has been 

 relatively reduced, and the development of spores 

 delayed to a late period, but special spore-bearing 

 organs have been evolved — the sporangia and 

 sporangiophores. Thus, on this theory, the so-called 



♦ See Bower, " Origin of Land Flora. 



