August 23, 1906] 



NA TURE 



429 



plastic nalure of nervous tissue renders it, in accordance 

 with the principles of natural selection, particularly favour- 

 able for progressive change in this direction, and thus 

 developments may occur which reach their highest physio- 

 logical expression in the brain of man. 



In conclusion, attention may be drawn to the peculiar 

 instability of living (irocesses and structures. The living 

 units show that significant mutability which the physio- 

 logist describes as metabolism. This mutability appears to 

 be encouraged or discouraged by the extent to which it 

 fulfils a purpose, and this purpose in a living organism is 

 the dominating law of its own development. The fulfil- 

 ment of this purpose by means of physical and chemical 

 change is such a general characteristic of living processes 

 that a physiologist may with some confidence suggest that 

 this fulfilment is the distinctive mark of a living thing. 



SECTION K. 



Oi'i:\iNc, .Address by Prof. F. W. Oliver, M..\., D.Sc, 

 F.R.S., Presiuient of hie .Section. 



The Seed, a Chapter in Evolution. 

 .\s the subject of the first portion of my .Address I propose 

 10 consider the place of the seed in the evolutionary historv 

 of plants. The seed-character is the distinctive mark of 

 three great groups of plants — the Pteridosperms, Gymno- 

 sperms (including Cordaile.-c), and .Angiosperms. Nor will 

 it be .seriously questioned that the possession of this organ 

 has given supremacy to seed-bearing plants over groups 

 not thus characterised in a majority of the types of environ- 

 ment where vegetation is able to exist. Exceptions, of 

 course, there are, though few of them are wholly immune 

 from the invasion of the Spermophyte. The sort of habitat, 

 for instance, in which Zostera flourishes — sometimes to the 

 exclusion of other forms — is held more as a result of 

 vegetative aggressiveness than in virtue of any special 

 power conferred by the seed-habit. 



Our stock of knowledge of those plants which had 

 attained to the seed-bearing condition in a bygone age has 

 undergone some extension during the last few vears ; the 

 seed, too, has shed its glamour over other bi^anches of 

 morphological inquiry, so that no serious apology is neces- 

 sary for its selection as the subject of this morning's 

 discourse. 



It is generally conceded that the primitive vegetation 

 arose in the waters, and that with the parting of the 

 waters and the emerging of land and continents this 

 primitive stock of plants was sufiiciently plastic to take 

 .idvantage of the new conditions, throwing up successive 

 hordes which effected a footing on the land, and in time 

 peopled the whole earth with forms adapted to the varying 

 habitats and climates as they differentiated. 



Of the character of these primjeval aquatic types no 

 direct information has been vouchsafed. It is a matter of 

 inference that they possessed much in common with the 

 green .Algse of to-day, which, living in a biologically stable 

 medium, are commonly regarded as their nearest represent- 

 atives. Be that as it may, the complexity of the life- 

 history of existing .Alga;; and the frequent presence of 

 neutral generations seem significant of the capacity of 

 their progenitors to originate forms with sporophytes 

 adapted to terrestrial conditions. 



In our Liverworts and Mosses on the one hand and the 

 Ferns and their allies on the other, two divergent evolu- 

 tionary lines are represented, both fitted to existence upon 

 land surfaces, but handicapped by the retention of a non- 

 terrestrial method of effecting the sexual process. In the 

 Bryophytes the physiological continuity and dependence of 

 the sporophyte upon the gametophyte is preserved through- 

 out, and it never rises above the status of an elaborate 

 spore-capsule ; whilst the gametophyte, though often reach- 

 ing a complex vegetative differentiation, offering many 

 analogies with the sporophytes of higher plants, is con- 

 demned to pigmy dimensions through the incubus of the 

 inherited aquatic mechanism of fertilisation. 



Though remote from the series that have culminated in 

 seed-plants, the Bryophytes are a group offering many an 

 Instructive parallel with the main series of plants ; certainly 



NO. 1921. VOL. 74] 



these forms have remained too long a thing apart. Haber- 

 landt and Goebel have shown us — to name no others — how 

 happy is the hunting-ground which the Bryophytes provide. 

 Further work is still required, directed more especially to 

 certain important points in the life-history. 



With the regular vascular cryptogams the relations 

 between the stages are of course different. Here we find 

 large complex sporophytes holding the ground, but 

 hampered by the ever-recurring necessity of dependence 

 upon outside water for the performance of the reproductive 

 process. 



The land problem was solved on ingenious lines. The 

 differentiation of gamctophytes which accompanied helero- 

 spory rendered possible the retention of the larger spore 

 and female prothallus. Thus retained aloft, the drawback 

 of the double existence is overcome and the advantages of 

 the elaborated sporophyte more fully realised. The water 

 conditions are brought directly under the plant's control 

 through the device of the pollen-chamber, and the way 

 paved for the ideal seed with siphonogamy. 



All the elements of the seed were present before, but 

 combined compactly in this new way we recognise what is 

 virtually a fresh stage intercalated in the life-history. 

 Further elaboration came bit by bit as (he possibilities 

 were successively realised. With the evolution of the .seed, 

 the plant rose at a bound to a higher plane, and this struc- 

 ture in its perfected form has become the very centre of 

 the plant's existence. 



The case of Cycas and Ginkgo with motile sperms affords 

 an extreme demonstration of the inertia of heredity, the 

 persistence in living seed-plants of the original aquatic 

 flagellate type. 



Obsolete as they are and faced with extinction, these 

 survivors from the middle epoch of the world's history 

 still hold their ground in a few scattered localities. In 

 this connection we shall listen with interest to Prof. 

 Pearson's account of the Encephalartos-scrub of South 

 Africa which is to occupy us during the course of the 

 present sitting of the Section. 



How the sperms became replaced ultimately by the 

 passive cells of the pollen-tube we have no knowledge. 



If the conjecture be well founded that the change came 

 late rather than earlv, then the conservatism of the spermo- 

 phytic line in this respect stands in marked contrast to the 

 adaptability that is so characteristic of another phylum of 

 aerial plants. The ready evolution of siphonogamy in the 

 form of fertilising tubes, so common in the Fungi, perhaps 

 finds its explanation in the close filiation of this group 

 with primitive and plastic forms. The fertilising tube may 

 reasonably be regarded as a special case of a general 

 susceptibility to chemiotactic stimuli which distinguished 

 the whole hyphal complex of the group from very early 

 times. In the case of the spermophyte, on the other hand, 

 the motile spermatozoid seems to have persisted through a 

 long and complicated ancestral history, so that its elimin- 

 ation may have been less easy of achievement. 



The seed, once evolved, became the centre of a host of 

 accessory organs, constituting what we know collectively 

 as the fruit and flower. By these it has been robbed, as 

 we shall see, of many of its pristine functions, and at the 

 same time has undergone marked structural reduction. 

 In the highly elaborated .Angiosperm more especially we 

 find an almost stereotyped uniformity in seed-structure 

 contrasting with an infinite diversity in the outward floral 

 husk. 



In attempting a sketch of the origin of the seed one 

 has to admit at the outset that recent discoveries bring 

 us no nearer to its prototype than we were a decade ago. 

 For the seeds of the Pteridosperms are advanced structures 

 recalling quite vividly the type long familiar in living 

 Cycads. It would be overstating the case to say they have 

 nothing primitive aliout them, but there is a long chapter 

 in evolution to be deciphered before we can connect, say, 

 the seed of Lyginodendron with the sporangium of any 

 Fern at present known to us. 



The great interest of the recent correlation of seeds with 

 Coal Measure plants lies less in the structure of these 

 correlated seeds than in the very extensiv series of plant- 

 remains which we have thus come to recognise as belonging 

 to the earlier Spermophvtes. 



For the position of these plants had remained in suspense. 



