October 



i 



1898. 



6^/ RDENERS' MA GA ZINE. 



635 



HOMOLOGY OF PLANTS. 



By Professor F. O. Bower, D.Sc, F.R.S. 



(Continued from page 621.) 



Alternation. 



t . nllP c t ions already discussed are mere matters of detail, compared 

 Bi ; r t q ! e at enigma of the alternation of generations m green plants, 

 * l V Lotion at large. This is, after all, a question of degree of 

 f m° ncnJ of the parts only, but of the whole plant or - gene- 

 homology, ^ test of all adaptations was really initiated we 



mm \ pxoect to bring to the point of demonstration ; at best we can only 

 Zure opinions of probability. Still this discussion commands at pre- 

 SeTmore widespread interest among botanists than any other in the 

 where of plant morphology. 



There was a time when the attempt was made to reduce all plants to 

 nne scheme as regards their life-cycle, a method which not only prevented 

 aLticity of theory, but was responsible for some unfortunate com- 

 parisons It was characteristic of the period when the text-book of 

 Sachs reigned supreme ; we find it there definitely laid down that " the 

 doctrine of alternation has the object of reducing to one scheme the main 

 phases of the life of all plants which bear sexual organs." But the con- 

 troversy between Pringsheim and Celakovsky had, as one of its results, 

 the recognition of various types of life-history, not of one scheme only. 

 The tendency at present is towards the opposite extreme ; the frequency 

 of the parallel developments now recognised has led some to accept a 

 comprehensive polyphyletic view as regards alternation, and wherever 

 difficulties of comparison arise, to take refuge in the plausible suggestion 

 that the organisms compared represent altogether distinct lines of descent. 

 But the view which should be confidently upheld is, that even where this 

 may actually be the case useful comparisons may yet be made ; and that 

 the method of progress within one phylum may illustrate the probable mode 

 of progress in another. The green Algre may thus throw light upon the 

 probable origin of the sporogonium in the Bryophytes, though they may 

 in no sense be in the line of their descent ; the Bryophytes may suggest 

 valuable ideas for the comparative study of the Pteridophytes, though 

 they may not represent their actual ancestry. 



It is the alternation as seen in these green plants that I propose to 

 discuss. Writers have distinguished various types of alternation, in- 

 cluding under the term divers modes of " alternation of shoots * ; and it 

 should be remembered that this was the original sense of the word 

 alternation as applied by Steenstrup. But gradually the issue in the case 

 of green plants has been simplified, and the question now centres round 

 that alternation of phases which some of us describe as " antithetic," 

 while others believe the phases to be really " homologous " as regards 



r - 



it 



iporopon^ m Coleochcete are frequently quoted as prototype 

 Sestivl f • °', Vn P° sltion has been that they may be " accepted as 

 PUnts'' 0 ,i Slmilar P r °gress in the course of evolution of Vascular 

 this • n .; he lf assum P t, on that the zygote is equivalent in all cases— 

 Funei woi M II a pure assumption— the fruit-body of such Alga? or 

 **** be rU i com P arable to the sporophyte in higher forms ; but it 

 H«r r n remembered that it is not even then proved to be 



unon' fk 1 has based a strong line of criticism of anithetic 



^cthinVrnm e f \ < i ases - He remarks : " The sudden appearance of 

 **>ry h ^ om P Iet ely new in the life-history, as required by the antithetic 



Wc aienotac 0 ^ m,nd ' a certain improbability. Ex nihilo nihil fit. 

 *Ppearinkr liU USt ° m f d in natura l history to see brand new structures 

 NaiUr « is conse m ° rP logical Me lchisedeks, without father or mother, 

 ^^one knows End when a new or & an is to be formed it is, as 



nce 1 feel r° S * ? lwa y|, fasbioi } e ^ out of s °nie pre-existing organ. 



writers on anithetic alternation, have been quite misunderstood. I have 

 contemplated no sudden development — indeed, on the first page of my 

 "Studies" I have spoken of the sporophyte as "gradually" interpolated. 

 Nor is the suggested development something " completely new," for I 

 specially speak of elaboration of the zygote. This is the parent 01 these 

 "morphological Melchisedeks " ; and unless segmentation be held to be 

 synonymous with " special creation," I confess I do not see where the 

 initial difficulty arises. I agree that Nature is conservative ; what we 

 contemplate is the fashioning of the sporophyte by a process of which 

 the first step is segmentation, out of a pre-existing organ — the zygote. 

 Such simple segmentation is seen in the case of certain Algse and Fungi, 

 and these may be taken as suggesting how the sporophyte of the Arche- 

 goniate may have come to be initiated. But I am not aware of having 

 ever suggested that these segmented zygotes of Algas are the homogenetic 

 prototypes of the more elaborate sporophytes. 



Dr. Scott further states that "the reproductive cells produced by the 

 ordinary plant of an (Edogonium are identical in development, structure, 

 behaviour, and germination with those produced 

 Professor Marshall Ward, 



by the oospore." 

 also speaking of (Edogonium, remarks, 

 "the attempt to get over this 



eir origin. 



Briefly put, the question is, How was the first start made ? Has the 

 eutral generation or sporophyte been the result of change of any other 

 part of the sexual generation than the zygote itself? If so, the alternation 

 is d homologous generations ; if not, then the alternation is what is styled 



m *\! l v lC * ■ ^ e w ^°^ e discussion is like a purely historical inquiry, but 

 with the minimum of documentary evidence ; for on this point the fossils 



ve scanty help. In the absence of more direct evidence we are thrown 

 m k on other arguments, such as those based on comparison of normal P| ants ; 

 specimens, and secondly upon the study of abnormalities. I shall not 

 attempt to treat the matter exhaustively ; it will, however, be necessary 

 or me to deal with certain points in the discussion which were raised in 

 we able address of Professor Scott at Liverpool. He there restated 



ingsheim s view of homologous alternation as against the anithetic. 



propose now to consider three matters which I think are most material 

 ° we discussion, viz. : (1) the bearing of the Alge and certain Fungi on 



•r™m Ue ?r 0n ; ^ tbe comparison from the Bryophyta ; and (3) the 

 ^ument from abnormalities. 



I. Alg/e and Fungi. 



subdili J St Si ? h u those A1 S^ and Phycomycetous Fungi which show a 

 start of L t ?i Zygote a PP ear ^ offer the key to the enigma of the first 

 of (7 / 1C a J ternatl on, and such rudimentary fruit-bodies as those 



by terming asexual spores borne by 

 the gametophyte gonidia, and reserving the term spore for bodies indis- 

 tinguishable from those gonidia by any morphological or physiological 

 character whatsoever, beyond their origin from a so-called sporophyte, 

 carries its own refutation." Now, as a matter of fact, Pringsheim's 

 description and figures of (Edogonium give scanty details ; in most of 

 the germinating zygotes the nuclei themselves are not clearly shown ; 

 much less the details of behaviour of those nuclei on germination. 

 Klebahn has described the fushion of the sexual nuclei in (Edogonium, 

 but I am not aware that he, or anyone else, has yet made detailed 

 observations 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 Professor Ward. We now know that important nuclear 

 changes do take place on the germination of the zygotes of certain Alg;« 

 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 (Edogonium ; the details may differ, but in the zygotes 

 of Closterium 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 Spirogyria. When it is further stated that in 

 some of these cases there is good reason to think that a reduction ot 

 chromosomes is connected with the division into four, just as a reduc- 

 tion is now known to accompany the tetrad division in Archegoniate 

 and Phanerogamic plants, it is plain that such cases as that of 

 (Edogonium ought not to be assumed to support a 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 generalization first stated by Overton, and 

 elaborated by Strasburger as regards the Archegoniate and Phanerogamic 



It has now been seen in cases drawn from various groups, that 

 the ceils of the gametophyte show a certain number (n) of chromosomes, 

 while those of the sporophyte show on nuclear division double that 

 number (2n) of chromosomes: Since Section K has had the advantage 

 of a statement on this subject from Professor 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 number of chromosomes 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. 



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

 of Strasburger, that the nuclear conditions of Fucus are peculiar ; that the 

 reduction only takes place on the formation of the sexual organs them- 

 selves, 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 adherents 

 of the antithetic theory views from which I personally dissent, has landed 

 Aem iSf^iflg nductio cui aburdum. He himself does "not think 

 we Tre as y "S i position to draw any morphological conclusions from 

 Ae^e minute differences, interesting as they are." But we need not accept 

 either of these extreme positions, if only a certain elasticity of heory be 

 main ainedf which should come naturally to adherents of polyphyletic 

 K£5m& I think the difficulty will chiefly be felt by those who like 

 enmp nf the earlier writers on alternation, attempt to reduce all plants 

 whTch show sexua ity n to one stiff scheme ; this has been found to fail 

 to the l^ti^ttrnltion, and a healthy recognition of various types of 

 u Ltfon L S the consequence. So in the matter of chromosomes 

 fTotTe posit o^Jhich the 2vent of reduction holds in the life cycle ; 

 and of theposuionun ^ ^ ^ may be anticipated, if we assume that 



Hut Strasburger has not considered 



H! nc * I feel \ Zl aivVtl >^ iasnionea ot some pre-existing organ. " uy- - ^"; fornl t0 one plan. But Strasburger has not considered 



^nce 5 an ; nt^i" difficulty in accepting the doctrine of the all plants 



^ n answer^ ]t ne ^!^ investigated do not fail in with a 



■Tpothesis starT ' state that t0 me the zygote, from which our conclusions, 

 ?* lib ilities I ' lS n0t nothin g " ; it is a cell with all the powers and 

 ff» deluded th?t^ Ple r e celL V,,chtin ^ in his«Organbildung,"has 



investigated do 



of comparison of behaviour 



.»T - 1 * * soerifir j a *i wm g vegetative cell which is capable of growth 

 J* cells typirX unalterable function. I have myself demonstrated 

 Slf^'y that t : Sporo ^ enou ^ may develop as vegetative tissue, and 

 5*°**iK)us. \v >SUes nor mally vegetative may on occasions become 

 that M ' a liv^ may ,' therefore , say generally as regards the sporo- 



fi^ ltcr ^le ^P?We of growth has not a specific 



zygote *nA ArHS 1 concer 



lhi *k thkt 5? ,tS early P roducts - 

 l °at the word*: « ;«£ >; 



fhis I conceive to have been the condition 



words " intercalation '* or " interpolation, J as used by 



because all plants 



K^nf.Hel beforTand a^ter ^fugltion \ti Closterium, Cosmarium 



certaTn Diatoms, and finally in ActinothrysM has arrived 

 t CfrnnrhVsion 44 that a shifting (Verschiebung of the time of division 

 Sto foS SS*Sr with reduction is possible in the history of develop- 

 / Tnrcrf nisms " It will doubtless be necessary later to put a precise 

 ^^SS^wofd * Verschiebung," and to define how far, in given 

 Sses it is ^o be understood as an actual shifting of the event within one 

 line of descent, how far it merely expresses an initial difference mam- 

 tained, or, it may be, extended, in different lines. 



W ' ' (To be continued.) 



