NovEMr.ER 24, 189S] 



NA TURE 



Dr. Scott further states that "the reproductive cells produced 

 by the ordinary plant of an Oedogonium are identical in develop- 

 ment, structure, .behaviour, and germination with those produced 

 by the oospore." Prof. Marshall Ward, also speaking of Oido- 

 goiiium, remarks " the attempt to get over this by terming 

 asexual spores borne by the gametophyte goiiidia, and reserving 

 the term sfore for bodies indistinguishable from these gonidia by 

 any morphological or physiological character whatsoever, beyond 

 their origin from a so-called sporophyte, carries its own refut- 

 ation." Now, as a matter of fact, Pringsheim's description and 

 figures of Oedogonium give scanty details; in most of the germinat- 

 ing zygotes the nuclei themselves are not clearly shown ; much less 

 the details of behaviour of those nuclei on germination. Klebahn 

 has described the fusion of the sexual nuclei in Oedogonium, but I 

 am not aware that he, or any one else, has yet made detailed ob- 

 servations on the nuclear condition of the zoospores, or the 

 changes which take place in the germinating egg. Till this is 

 done I submit that it is premature and undesirable to make such 

 assertions as those of Dr. Scott and Prof. Ward. We now 

 know that important nuclear changes do take place on the 

 germination of the zygotes of certain Algae and Fungi. These 

 changes are connected with a division of the nuclei into four, 

 which is the number of the zoospores usually produced on 

 germination in Oedogonium ; the details may differ, but in the 

 zygotes of Closlerium and Cosmarium, and in the formation of 

 the auxospores of Rhopalodia, Klebahn has demonstrated this 

 division into four ; also Chmielewsky has described a similar 

 production of four nuclei in the germinating zygotes of Spirog)'ra. 

 When it is further stated that in some of these cases there is 

 good reason to think that a reduction of chromosomes is con- 

 nected with the division into four, just as a reduction is now 

 known to accompany the tetrad division in Archegoniate and 

 Phanerogamic plants, it is plain that such cases as that cf 

 Oedogonium ought not to be assumed to support an homologous 

 view without any fresh observation of the facts. 



With the whole question of alternation, the nuclear details 

 and differences in number of the chromosomes on division are 

 now intimately bound up. Though the observations are still 

 few, so far as they go they are consistent with the generalisation 

 first stated by Overton, and elaborated by Strasburger as regards 

 the Archegoniate and Phanerogamic plants. It has now been 

 seen in cases drawn from various groups, that the cells of the 

 gametophyte show a certain number («) of chromosomes, while 

 those of the sporophyte show on nuclear division double that 

 number (2«) of chromosomes. Since Section K has had the 

 advantage of a statement on this subject from Prof. Strasburger 

 himself at Oxford, and as Dr. Scott also discussed the matter at 

 Liverpool, I need not enlarge. I shall only remind you that 

 Strasburger took up the position that the numfier of chromo- 

 somes which appears in each sexual nucleus is that original 

 number which the ancestors possessed in a pre-sexual period ; 

 while the reduction of the double number which results from 

 sexual fusion is, in his opinion, to be regarded as an atavistic 

 process. As far as investigation has yet gone, I see nothing to 

 prevent the acceptance of this as a provisional theory. 



It is now well known, however, from the observations of 

 Farmer and of .Strasburger, that the nuclear conditions of Ftuus 

 are peculiar ; that the re<luction only takes place on the form- 

 ation of the .sexual organs themselves, and that the Fucus plant, 

 like a sporophyte in the Archegoniate series, has the double 

 number of chromosomes. At first sight this might appear to be 

 a fatal difficulty, and Dr. Scott, attributing to the adherents of 

 the antithetic theory views from which I personally dissent, has 

 landed them in a seeming reductio ad absurdum. He himself 

 does " not think we are as yet in a position to draw any morpho- 

 logical conclusions from these minute differences, interesting as 

 they are." But we need not accept either of these extreme 

 positions, if only a certain elasticity of theory be maintained, 

 which should come naturally to adherents of polyphyletic de- 

 velopment. I think the difficulty will chiefly be felt by those 

 who, like some of the earlier writers on alternation, attempt to 

 reduce all plants which show sexuality to one stiff scheme ; this 

 has been found to fail in the case of alternation, and a healthy 

 recognition of various types of alternation has been the con- 

 secjuence. So in the matter of chromosomes, and of the position 

 which the event of reduction holds in the life-cycle ; difficulties 

 such as this in Fucus may be anticipated, if we assume that all 

 plants will conform to one plan. But Strasburger has not 

 considered it necessary to cast aside the nuclear details as a 

 basis for morphological conclusions, because all plants investi- 



NO. T517. VOL. 59] 



gated do not fall in with a preconceived scheme. On grounds 

 of comparison of behaviour of the nuclei before and after 

 conjugation in Clostcrium, Cosmarium, Spiro/>yra, in certain 

 Diatoms, and finally in Actinophrys, he has arrived at the con- 

 clusion " that a shifting (Verschiebung) of the time of division 

 into four, together with reduction, is pos.sible in the history of 

 development of organisms." It will doubtless be necessary later 

 to put a precise meaning upon the word " Verschiebung," and 

 to define how far in given cases it is to be understood as an 

 actual shifting of the event within one line of descent, how far 

 it merely expresses an initial difference maintained, or it may 

 be, extended, in dift'erent lines. Meanwhile, those who accept 

 Prof. Strasburger's position will see that while in various 

 evolutionary sequences the reduction may take place at different 

 points in the cycle, still it may have settled down to a fixed and 

 constant position in any one sequence ; that I conceive to have 

 been the case for the Archegoniate series. The validity of this 

 conclusion does not seem to me to be affected by the diverse 

 state of things seen in so far removed a sequence as that of the 

 brown Algae. 



Here a brief reference must be made to the very beautiful 

 results of W'ager on the changes in the zygote of Cystopus can- 

 didus, which have been verified and extended by Berlese. 

 Wager states that in this fungus the process of fertilisation does 

 not differ in any essential particular from the process as it takes 

 place in Angiosperms. On the division of the fusion-nucleus of 

 the zygote the number of the chromosomes present before 

 division appears to be considerably in excess of the number 

 observed in the nuclei of the oogonium. " By counting as care- 

 fully as possible 20 to 24 or even more appear to be present, 

 and the impression is produced that the number^ is certainly 

 much larger than that observed in the oogonium." Divisions 

 of the nucleus then follow to form 4, 8, 16, and finally 32, in 

 which condition a period of rest ensues ; and finally, it appears 

 that a division of each into four follows, to form the nuclei of 

 four spores. Wager believes the reduction to take place at this 

 last division, and Berlese has established a strong probability 

 that such a reduction actually does take place. Plainly these 

 observations are not final or conclusive, and even if they were, 

 the strict homogeny of this fruit-body with a rudimentary sporo- 

 phyte of a green plant would not be proved. It must, however, 

 rank at least as an important parallel case, illustrating how the 

 reduction may be effected in a distinct line of descent. 



We see then that in green Algae such as Oedogonium, 

 Sphaeroplea, and Cokochaele, certain divisions follow fertilisation, 

 but we are not yet in possession of the nuclear details. I prefer, 

 therefore, to suspend judgment as to the nature of those divi- 

 sions ; but in view of the peculiar behaviour already seen in 

 other zygotes, it may be distinctly anticipated that some form of 

 reduction will be demonstrated at that stage. If that be shown 

 then we shall be right in recognising in these small cell-bodies 

 the rudimentary correlative of a sporophyte — the sort of begin- 

 ning from which a neutral generation may have sprung in land- 

 living plants. We cannot go further than this as regards the 

 green Algae until we are in possession of the facts. There is no 

 greater desideratum in morphology at the present moment than 

 a detailed knowledge of the germination of zygotes such as that 

 of Otdoganiuin. 



Here I may remark that the admirable observations of Prof. 

 Klebs, whom the Section will welcome as a distinguished guest, 

 do not appear to me to touch this question. His very varied 

 and convincing experiments show in a number of Algae and 

 Fungi that, as regards the succession of vegetative and sexual 

 modesof propagation, theexperimenterhasa very- com pletecontrol. 

 I do not find, however, any observations of his which touch the 

 behaviour of germinating zygotes of green Algae as regards 

 details of segmentation. I do not mention this as in the least 

 impairing the brilliancy of Prof. Klebs's work, but because Prof. 

 Ward has brought Klebs's results to bear upon the discussion on 

 antithetic alternation in a manner which I do not think that the 

 facts will support. 



II. Biyophyta. 



Turning now to the Bryophytes, these plants stand at the 

 moment in a somewhat discredited position. We have been 

 warned by Dr. Scott that "there is no reason to believe that 

 the Bryophyta, as we know them, were the percursors of the 

 Vascular Cryptogams at all," and that " there is no appreciable 

 resemblance between the fruit of any of the Bryophyta and the 

 plant of any Vascular Cryptogam," and the suggestion has been 



