PROBLEMS OF PLANT LIFE. 



II. THE ORIGIN AND EVOLUTION OF THE HIGHER PLANTS. 

 By S. REGINALD PRICE, M.A. (Cantab). 



In a previous article * some short account was 

 attempted of the possible lines of evolution which 

 may have resulted in our present Algal flora, and 

 it was there mentioned that there is good reason to 

 suppose that some such Alga-like plants represented 

 the earliest members of the plant kingdom. If this 

 were the case the so-called " Higher Plants " must 

 have descended from these Algal-like ancestors. 

 As was there indicated also, the Flagellata are 

 probably to be regarded as a plexus of organisms, 

 which represents the nearest approach that we can 

 imagine to the primitive group of both animal and 

 vegetable life. Some of the processes which probably 

 took place in the evolution of the Green Algae were 

 briefly outlined and the question now arises — 

 WTiat has been the course of evolution from these 

 lowly plants to the Higher Plants that we know 

 to-day ? An answer to this question is still 

 impossible, but biological speculation has been 

 rife on the subject. It may thus be of interest to 

 state in outline some of the main bearings of the 

 problem, and to indicate briefly to what extent 

 our present knowledge can deal mth it. 



The organisation of the highest plants of which 

 we are speaking is very much more complex than 

 that of the simple water forms existing, for example, 

 in the Green Algae, and at first sight the difference 

 seems too great for any relation whatever to be 

 possible between the two. However, in the Algae 

 themselves, especially in the " coloured seaweeds," 

 there are numbers of species existing which show 

 a high organisation of body structure, and indicate 

 at once the possibility of upward evolution for the 

 group. Such forms, for example, as Fucus, the 

 Bladder-wrack, and Laminaria of the brown 

 seaweeds (Phaeophyceae) show a differentiation of 

 the body into fixing and assimilating or leaf-like 

 portions, and have organised definite conducting 

 and assimilating tissues. In fact, in organ- 

 isation of the body some of these higher seaweeds 

 seem to be more advanced than some of the lower 

 liverworts, a section of the Bryophyta which must 

 be regarded as][the lowest true' group of the land 

 flora. 



The most far-reaching and important change 

 which we have to trace is that of habit, from 

 primitively aquatic to land-living forms ; and still, 

 at the present day, a large number of land types 

 retain characters which are only to be regarded 

 as descended from aquatic ancestors. Thus, for 



example, the motile male cell is universal through- 

 out the Bryophyta and Pteridophyta, so that at 

 the time of fertilisation an aquatic en\-ironment 

 is necessary, though this need be no more than a 

 film of dew or a rain-drop ! 



The Higher Plants, which constitute very largely 

 the terrestrial flora, comprise practically the three 

 great groups : the Moss Plants or Bryophyta, the 

 Fern Plants or Pteridophyta, and the Seed Plants 

 or Spermaphyta, which include the flowering plants. 

 A few of these are, of course, aquatic in habit, 

 but there is every reason to believe that they 

 are only secondarily so, and have descended from 

 previous land-living ancestors. 



These great groups as they exist to-day are 

 sufficiently sharply differentiated in character from 

 one another. There is no space here to deal \\ith 

 the features which characterise the groups — types 

 from each will, at any rate, be familiar to every 

 biologist. One or two features which are of great 

 importance must be emphasised in considering this 

 problem of evolution. 



The first is bound up in the phenomenon of 

 Alternation of Generations. Suppose we consider 

 a plant bearing sexual organs, the egg after fertilis- 

 ation — the oospore — may germinate to produce a 

 plant like the first, or it may produce another 

 phase or generation which bears only asexually 

 produced spores. Generally speaking, in the Higher 

 Plants the latter is the case, and the life-cycle is 

 more or less obligate f — the oospore produces the 

 spore-bearing phase or " Sporophyte," and the 

 spores on germination produce the sexual organ- 

 bearingphase or" Gametophyte." Thisis illustrated 

 in the diagram of the normal life-cycle of a common 

 moss, say Funaria or Polytrichum (see Figure 164). 

 This regular alternation of phases is what is called 

 " Alternation of Generations in the Higher Plants." 



In the Bryophyta the sporophyte is represented 

 by the capsule and stalk, or sporogonium, which is 

 produced as a development of the oospore and 

 remains dependent on the gametophyte. The 

 gametophyte is the real free-living and self-support- 

 ing generation. In the Pteridophyta almost the 

 reverse is the case, for the sporophyte is independent 

 after its embryonic stage, while the gametophyte 

 is a little, short-lived, independent structure — the 

 prothallus. In some cases there are two gameto- 

 phytes which bear the different sexual organs. 

 There is thus a sharp contrast between the two 



• " Knowledge," June, 1913, " Evolution among Lowly Forms (the Algae)." pages 201-204. 

 t See, e.g., Bower, " Origin of Land Flora." 

 B 179 



