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GARDENERS' M^4 GAZINE. 



October 29, 1898 



HOMOLOGY OF PLANTS. 



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



( Concluded from page 684 . ) 



Dorsiventrality. 



Inieresting questions arise in connection with dorsiventral structure. 

 In the Equisetine^, and almost all Lycopodinece, the strobilus is of the 

 radiate type, therein corresponding to the radial structure of typical 

 sporogonia. While certain ferns are of the radiate type, others are con- 

 spicuously dorsiventral, even from their earliest embryonic state. Dorsi- 

 ventral structure also appears in the vegetative region, and sometimes, 

 though rarely, in the strobilus of Selaginella. Professor Goebel, in a 

 chapter of his " Organographies the publication of which may be recog- 

 nised as the leading event in the morphological studies of the year, dis- 

 cusses the origin of the dorsiventral state in a number of examples, and 

 his results have a most interesting bearing on our theory of the 

 strobilus. 



He shows in the case of Vaccinium myrtillus how the first shoot of 

 the seedling is orthotropic and radial ; the lateral shoots, formed after 

 the apical growth of this is arrested, are also orthotropic, but the lateral 

 shoots of higher order become plagiotropic with leaves in two lateral 

 rows. He points out the intermediate steps from one condition to the 

 other, and how finally the growing point itself is influenced by the 



systematists and morphologists both find their greatest difficultv in rti 

 incompleteness of the record, and the frequent isolation of the th\*Ll . ♦ 

 be classified. nmgs to 



review 



r, we may now briefly 



lew our position as regards organography, and the following cate- 



gories are to be recognised, though they graduate almost imperceptibly 

 into one another : — r / 



external agency (apparently light), which leads to a change of the leaf possibilities : 

 arrangement. This seems to be the case in many other Phanerogamic 



Homogeny.—{a) Repetition of the individual part in successive 

 generations, with the same number and position. This is exemplified bv 

 the cotyledons, the foot, and the first root. y 

 (6) Essential correspondence of parts varying in number and position 

 but corresponding in character and development, produced in a regular 

 sequence ; e.g., most cases of continued embryology. 



(c) Transferred position of parts, similar in origin and structure to 

 those produced in regular sequence ; e.g., roots, adventitious buds, sori 

 of Aspidium ano malum, aposporous and apogamous growths, many mon- 

 strosities ; these we may believe to result from a transfer of inherited 

 developmental capability. 



Homoplasy. — This may be recognised with varying degrees of proba- 

 bility ; starting from cases where the question of community of descent 

 is open (as with nearer circles of affinity), and proceeding to those in 

 which distinct evolution is virtually certain. It remains for future in- 

 vestigation to clear up doubtful points. Meanwhile, taking the case of 

 leaves for the purpose of illustration, we may contemplate the following 



plants. 



A particularly interesting account is also given of similar changes in 

 Selaginella. Some eight species are orthotropic, radial, and isophyllous. 

 S. sanguinolenta shows a direct response to external conditions, being 

 upright and isophyllous in bright and dry situations, plagiotropic and 

 anisophyllous in damp and shady situations. The bulk of the genus are, 

 however, either plagiotropic and anisophyllous throughout, or some may 

 have an early orthotropic stage. But he concludes that even in 

 "habitually" anisophyllus Selaginellas we have to do with an adaptive 

 character, induced probably by light. 



We see then good evidence that in certain cases the dorsiventral 

 shoot is a result of adaptation, and the radial probably the primitive. 

 Was this always so ? We need not discuss the case of the gametophyte, 

 as the problem there is even more varied and difficult, and does not at 

 the moment engage our attention. But the question whether in the 

 sporophyte the radial was in all cases the primitive type is clearly related 

 to our theory of the strobilus. The sporogonia of Bryophytes are, with 

 few exceptions, orthotropic, and almost uniformly radial ; exceptions, 

 such as IHphyscium and Buxbaumia have been shown to have an inter- 

 esting relation to the incidence of light, and are readily recognised as 

 derivative. The distinctively strobiloid Pteridophytes mostly maintain 

 this radial structure ; this may be so both in strobilus and vegetative 

 organs, as in Equisetum, Isoetes, in most species of Lycopodium, and in 

 some Selaginellas ; or the vegetative region may be dorsiventral, and 

 the strobilus return to the radial type, as in some species of Lycopodium 

 and most Selaginellas ; but in some Selaginellas even the strobilus may 

 be dorsiventral. 



In the Ferns the case is less obvious ; the large size of the leaves, 

 combined often with a dorsiventral structure of the shoot, makes a com- 

 parison witharadial strobilus lesseasy. Goebel has pointed out that inmany 

 dorsiventral Ferns the dorsiventrality is already defined in the punctum 

 vegetationis, and does not depend upon a subsequent shifting of the parts. 

 But it should be remembered how many Ferns are orthotropic and 

 radial ; that almost all the large genera include species with simple 

 unbranched leaves. Further, the series of the Ophioglossace^e, possibly 

 a distinct phylum from the true Ferns, may be held to illustrate a progres- 

 sive elaboration of the leaf, from smaller-leaved forms which are ortho- 

 tropic and radial, to larger-leaved forms, which are sometimes ortho- 

 tropic and radial {Botrychium), sometimes plagiotropic, and dorsiventral 

 {Helminthostachys). It is not, I think, improbable that these, and also 

 the true ferns, are referable in origin to an orthotropic strobiloid type, 

 with radial structure. This opinion was in substance suggested in 1894 

 at Oxford ; these recent observations of Goebel on the derivative nature 

 of dorsiventral shoots strengthen the position then taken up, while they 

 supply us with fresh examples of homoplastic development. 



Conclusion. 



This discussion was entered on with a view to finding whether phy- 

 togeny as a basis of morphology would lead us. However unprepared we 

 may be to pursue it with certainty into detail, or to apply a terminology 

 to the sequences which we recognise, we must, I think, accept phylogeny 

 as the natural basis for morphology. I do not think that any middle 

 course between this and an artificial system is possible or reasonable. 

 But here we launch ourselves upon a sea of uncertainties on which we 

 must keep our course with care. Following it, we think we espy certain 

 great movements in Nature. We may recognise what we believe to be 

 a true evolutionary sequence, but who is to say whether it is a progres- 

 sive or a retrograde sequence? It may even be one divergent from some 

 middle point. Our best friend may read the sequence in opposite order 

 to ourselves and arrive at a diametrically opposite conclusion. There is 

 no finality to this judging of probabilities, a fact which should be always 

 before the mind, especially in the warmer moments of discussion. 



it is interesting to trace the parallel between the progress of classifi- 

 cation of plants as a whole, and that of the classification of their parts, 

 in each case the earlier symptoms were artificial. We may compare the 

 2? s .y st J em °f. taxonomy with the Hofmeisterian organography ; in 

 Dotn the rigid application of a preconceived method placed incongruous 

 things in juxtaposition, in each case a widening of the basis of the classi- 



Snt ?u i aS /f ed m a redistril >ution on more natural lines. The pre- 

 sent meal of taxonomy is the same as that of the phylogenetic organo- 

 graphy, viz., to group according to descent. The limitations are alike : 



(a) A possible origin of two homoplastic series of leaves in the same 

 plant, and the same generation {Phylloglossiwi). 



{b) Two homoplastic series in the same plant, but in different genera- 

 tions {Lycopodium cernuum). 



(c) A possible distinct origin of homoplastic leaves in distinct phyla, 

 but in the same generation (sporophyte of Ferns, Lycopods, Equiseta). 



(d) A distinct origin of homoplastic leaves in distinct phyla, and 

 distinct generations {e.g. leaves of Bryophyta and of Pteridophyta). 



Now Homology has been used in an extended sense as including 

 many, or even all, of these categories. It seems plain to me that this 

 collective use of the term homology carries no distinct evolutionary idea 

 with it ; it indicates little more than a vague similarity ; the word will 

 have to be either more strictly defined or dropped. The old categories 

 of parts based upon the place and mode of their origin are apt to be 

 split up if the system be checked by views as to descent. Comparison, 

 aided by experiment, supersedes all other methods, and the results 

 which follow raise the question of terminology of parts which have arisen 

 by parallel development. 



In parts which are of secondary importance, such as stipules, pinna*, 

 the indusium, hairs, glands, the inconstancy of their occurrence points 

 to independent origin by parallel development in a high degree ; in 

 parts of greater importance, such as leaves, a parallel development 

 may also be recognised, though in a less high degree ; in the case of 

 sporangia their acceptance as a category sui generis dispelled the old 

 view of their various origin from vegetative parts ; but we must remem- 

 ber that this does not by any means exclude a parallel development 

 also in them, by enlargement and septation from some simpler spore- 

 producing body, though this is not yet a matter of demonstration. 

 Finally, the sexual organs are probably homogenetic in all Archegoniate 

 plants, but we have no proof that sexuality arose once for all in the 

 lower plants ; the probability is rather the contrary. Thus we may 

 contemplate as very general a polyphyletic origin of similar parts 

 by evolution along distinct lines, but resulting, it may be, in forms 

 essentially similar. 



There are two extreme courses open to those who wish to convey 

 clearly to others such matters as these : the one is to use a separate term 

 for each category of parts, which can be followed as maintaining its in- 

 dividual or essential identity throughout a recognised line of descent— in 

 fact, to make a polynomic terminology of members run parallel with a 

 polyphyletic development ; the other course is to make it clear always in 

 the use of terms applied to parts that they do not convey any evolutionary 

 meaning, and to use them only in a descriptive sense. Perhaps trie 

 former is the ideal method, and it may be a desirable thing, as polyphy- 

 letic origins of parts become more established, that the terminology 

 should be brought to reflect at least the more important conclusions 

 arrived at. How this may be done we leave for the future to « ecia £ 

 though I have indicated a first step in the case of the leaves oi Mosses 

 and Ferns. . . 



But, for the present, the whole matter is still so tentative that it is wen 

 be content with something which falls short of the ideal, and to ma !™;\ 



s stem, leaf, root, hair, sporangium, 

 descriptive of parts which correspond as regards general teal " rc:> 

 origin, position, and nature ; but with no reference either, on tn 

 hand, to conformity to any ideal plan, or, on the other, to anv commun 7 e 

 by descent ; in fact, we shall preserve the original pre-Damiman s« 

 of these words, which was purely descriptive, and avoid any attempt 

 read into them any accessory meaning. . u . , ^velop- 



A special interest attends those cases of transfer of ^ he ™ ea rt Q _ rS b u t 

 mental capability where a part appears with its normal cha racici , } 

 in a position which is not usual, such as the transfer of the s 

 Aspidium anomalumj comparable with these transfers on tne u ja 

 are those apogamous growths where roots, leaves, ram ^*' F maV be 

 may arise independently out of the usual succession. *j? esc ' rtS| 

 compared, on the other hand, with those interpolations 01 cxi * 

 such as the accessory stipules in the stellate Rubiaccae, £ deter- 



in Rosacea , &c. We are unable as yet to say what t is wn ^ 

 mines the position and mode of origin of parts ; I "gSLTS material 

 that Sach's hypothesis of « Stoff und Form," involving ldeas oT tion 

 differences which have not been demonstrated, will advance u 1 ^ 

 so much as a careful following of the details in the ongino 1 

 say in some of these apogamous growths. Here we see inc f 



the usual terms, such as 



