68 



NA 'JURE 



[No\ EMBER i;, 1898 



as in the case of ihe Sunflower leaves, an essential corre- 

 spondence, but not an individual repetition of the parts, and we 

 learn that parts thus essentially corresponding to one another 

 may be '.ransferred to unusual positions. 



Secondly, in plants more or less nearly related, those which 

 are less akin may show so slight a similarity in detail that again 

 questions of the essential correspondence of the parts may arise. 

 Within nearer circles of affinity these questions will afi'ect only 

 the appendages of minor importance, which shosv less constancy 

 of occurrence and arrangement, such as emergences and hairs ; 

 but in case of plants less nearly akin the degree of correspond- 

 ence of the larger members may become a matter of debate. 

 Take, for instance, the three great phyla of living Pteridophytes, 

 the Ferns, Equiseta, and Lycopods. While the sporophyte as 

 a whole in each of these may be accepted as homologous by 

 descent wiih that of the others, the question as to the true corre- 

 spondence by descent of the leaves must still be open for dis- 

 cussion. It is a tenable viesv that the three phyla arose separately 

 from a non-foliar ancestry, and that the assumption of a foliar 

 development, having in each case a difl'erent habit, and a different 

 relation to the sporangia, led to the distinctiveness of the three 

 stocks. Opinion on the point of homology by descent of the 

 leaves of these Pteridophyta must at present remain in suspense ; 

 but the case is different with the leaf of Pteridophytes as com- 

 pared with the leaf of Bryophytes : unless the whole morpho- 

 logical system of the time be in error, we shall be right in 

 maintaining that these foliar developments have been distinct in 

 origin from the first. 



Now all the foliar parts above quoted would in a system of 

 merely formal morphology fall into the category of " leaves." 

 But if ph)logeny be accepted as the only real basis of morpho- 

 logy. We must be prepared to split up the category based on 

 mere time, place, and mode of origin, and to recognise in some 

 cases repetition of individual parts ; in others essential corre- 

 spondence, but not individual repetition, owing sometimes to 

 transfer of developmental capability ; in other cases again, a 

 possibility of distinct origin by descent not actually proved ; and 

 lastly a reasonable certainty of distinct origin. The practical 

 question for the morphologist is, having recognised these facts 

 for himself, how is the matter to be best made intelligible to 

 others ? 



A reconsideration of the term "Komology" will thus be 

 necessary ; is it to be applied equally to such parts as are 

 connected by lineal descent, and also to those which we have 

 good reason to believe have resulted from |}arallel development 

 in quite distinct phyla ? Or, to put a finer point upon our 

 inquiry, are we to distinguish in any way the cases of "in- 

 dividual repetition " from those of " essential correspondence " ? 

 In the latter case I think no good end would be served at 

 present by accentuating this distinction by terms : the steps of 

 divergence are so slight and gradual. None the less should it 

 be clearly borne in mind that comparisons of parts commonly 

 ranked as homologous in the plant body are based on a less 

 complete individual correspondence than that of parts usually 

 compared in the animal body. 



But the case is different in dealing with parallel develop- 

 ments, and some doubt arises whether parts which probably, or 

 it may be certainly, have arisen by separate evolutionary 

 sequence in distinct phyla are to be classed as homologous in 

 the same sense as those directly related by descent. This 

 question was long ago taken up on the zoological side by 

 Prof. Ray I-ankester, and it was shown that the old word 

 " homology " covered two things recognised as distinct from 

 the point of view of descent. He defined as hoinogeiioui 

 "structures which are genetically related, in so far as they 

 have a single representative in a common ancestor." On the 

 other hand, " when identical or nearly similar forces or en- 

 vironments act on two or more parts of an organism which are 

 exactly or nearly alike : further, if, instead of .similar parts in 

 the same organism, we suppose the same forces to act on parts 

 in two organisms, which parts are exactly or nearly alike, and 

 sometimes homogenetic, the resulting correspondences called 

 forih in the several parts in the two organisms will be nearly or 

 exactly alike. ... I propose to call this kind of agreement 

 lio'iii'plasi! or homoplasy. ' Now this distinction of terms 

 requires also to be observed in plant-morphology, and I am 

 surprised that it has never yet been adopted by botanists, 

 though we have long recognised cases of parallel development. 

 I do not propose now to spend lime in assigning these terms to 

 familiar cases : but to lake the examples already cited, the leaf 



NO. I 5 16, VOL. 59] 



of a Fern would be homoplastic, though not homogenetic with 

 the leaf of a Moss ; or, taking examples from plants more nearly 

 akin, it would appear pf)ssible that the leaves of the three 

 distinct phyla of living Pteridophytes show merely homoplasy 

 not a true homogeny. 



The successive foliage leaves of most plants are assumed in 

 the individual to be the result of a mere repetition of develop- 

 ment. But it is quite a possible view that in the plant-body (as 

 is contemplated in the animal in those cases of "serial homo- 

 logy" which Lankesler recognises as homoplastic) homoplasy 

 may have had a place. We must inquire whether all those 

 structures which we designate "leaves" have actually been the 

 result of a development identical, or at least essentially similar 

 as regards their origin in the race. The problem is, given a 

 plant with numerous leaves of various form and function, to 

 unravel the real story of their evolution. Two distinct factors 

 may be contemplated as possibly occurring even in the indi- 

 vidual, viz. : 



(1) Homogeny of genetically related parts, with or without 

 repetition of the parts formed. 



(2) Homoplasy, an origin of two or more distinct categories of 

 parts, not genetically related, on the same organism. 



Working upon either of these, and thus complicating the 

 problem by obliterating such distinctions as may have existed at 

 first, may be the phenomenon of iiielamorfhosis. This has lately 

 received its evolutionary definition at the hands of Prof. Goebel, 

 as restricted to those cases where there has been an obvious 

 change of function. We see how change of function accounts 

 for various forms of leaf in certain cases ; but it does not follow 

 that all leaf-forms on the same plant were so produced, by 

 metamorphosis of a single original type. 



The Lycopodine.ae are particularly interesting in illustration of 

 this point. It appears probable that Phyllogloisum is a more 

 primitive type than other living Lycopods ; it has two kinds of 

 leaf, the protophylls borne in irregular number and arrangement 

 on the protocorm, and the sporophylls of different form from 

 these, and arranged regularly on the strobilus : commonly there 

 are no intermediate steps between them. This condition in a 

 plant, which on general grounds of comparison we believe li' 

 be primitive, is certainly interesting, and we shall ask whether 

 the two types of leaf have not arisen by distinct evolutionary 

 sequence ? In the genus Lyiopodium there are certain species, 

 such as /.. Selago, which show alternately sterile and fertik 

 zones ; examining the limits of the sterile zones, we find at tht 

 base of each leaf an atrophied sporangium, similar in position 

 to that borne by a sporophyll. When we compare this con 

 dition with that of Phyllog'ossiim it appears probable that the 

 successive zones are the result of a metamorphosis of a strobilus. 

 which had a continuous apical growth, and unlimited repeti'ior 

 of sporophylls, but that some of these suft'ered atrophy of iheii 

 sporangia, with the correlative effect of a larger vegetative 

 development. A differentiation of the strobilus thus results in 

 the plant as we see it, a production of foliage leaves by sterili- 

 sation of sporophylls. Recognising this, some may suggest 

 that the protophylls originated in the same way. It is possibU 

 that they did ; but it is equally possible, and, in view of thi 

 peculiar case of Phylloglossum, I think more i)robable, that in 

 these plants we have an example of homopl.astic developmeni 

 of parts distinct as to descent, while the limits of the two still 

 evident in Phylloglosnim became obliterated in the more ccm- 

 plex case of l.ytopoi/ium. The proof of the point will be difti 

 cult or even impossible, but the eyes of botanists shouUi 

 certainly be open to recognise such individual homoplasy. 

 should it occur, and to inquire whether it has really had a place 

 in plant-development. 



Returning now to homoplastic development in distinc 

 groups of plants, the morphology of \.\\e/ool provides inlerestini; 

 material for comparison, and especially so since there is n> 

 question of repetition here; for the comparison is betweei 

 parts of which only one appears on each individual plant. 



The term foot has Iwen applied to that part of the embryo ir. 

 Pteridophyta which serves to connect it physiologically with tin. 

 prothallus ; the term has also l)een used for the b.ase of the set 1 

 m Bryophytes. Parts performing a similar function, but no; 

 referable, as in other Phanerogams, to the metamorphosis it 

 cotyledons, are also found in Gnelum and Wchk'tls<hia. 



In the Bryophyta what is usually called the foot is no definiteh 

 specialised structure ; it is merely the absorbent base of the seU' 

 It would appear probable that in the Bryophyta a true homogeny 

 holds in all cases, as the requirement for it will have bei" 



