November 17, 1898] 



NA TURE 



67 



from the point of view nf descent, now tends to supersede the 

 formal morphology of the second period, which in many minds 

 implied or assumed ideal types or creative plans. It has become 

 a general view that the facts of morphology are but the stereo- 

 typed facts of physiology, form being determined by function, 

 but under the check of heredity. This third experimental 

 phase of the study of plant form is directed, as it were, to the 

 very setting of the types, before the stereotype plate is cast. 

 We watch nature's compositor at work, but we also ascertain 

 that the plate itself, after it is cast, is much more plastic than 

 some of us had thought. 



These three phases of morphological inquiry have naturally 

 overlapped one another ; we recognise, however, that first 

 description, then formal comparison, and now experiment, have 

 been the leading features in morphological investigation during 

 these successive periods. 



Homology. 



The ideal aimed at in the study of the morphology of plants 

 is to trace their real relationships and mode of origin, on the 

 basis of the widest observation — in short, to reconstruct the 

 evolutionary tree. In order to make comparison possible, or at 

 , least manageable, a terminology is necessary, and this not only 

 1 of the plants themselves, but also of their parts. We may for 

 i, the moment leave on one side that summing up of morphological 

 , opinion represented by the systematic arrangement of plants in 

 ] a taxonomic system. I propose to-day to discuss not the 

 i( classification of plants, but the classification of the parts of 

 \\ plants, their grouping according to their homology. And here 

 I use a word which is probably explained to every class of 

 I elementary students ; it is one of those terms a meaning of 

 I which is indeed revealed to the babes of the science, while 

 ;l those who teach are not at one as to its definition. We need 

 ;i not enter now into the various opinions which have been 

 -I held on this point, nor need we make any antiquarian research 

 I into the introduction or early use of the word homology ; it will 

 I suffice to state that it was already firmly established in the 

 science before views as to descent gave it any intelligible 

 [, meaning. We speak of the homologies recognised by Hof- 

 I meister, but it should be remembered that their great discoverer 

 I did not put an evolutionary interpretation upon them. Sachs 

 I points out in his history how "the theory of descent had only 

 j to accept what genetic morphology had already brought to view." 

 I Nevertheless, nmch remained ingrained in the very texture of 

 I the science which was incompatible with evolutionary thought. 

 ( This was so even in the text-book of Sachs itself. The cate- 

 I gories of root, stem, leaf, and hair are there laid down, and 

 the parts classed under these several heads were held to be 

 homologous. In their definition all those characters which refer 

 J to function were put aside, the definitions relating to origin and 

 ' relative position : the reproductive organs were grouped with 

 ' the rest, with the result that these parts were described as bear- 

 j ing a varying morphological value. But this purely formal mor- 

 I phology is now dead ; it long survived a mere passive belief in 

 j evolutionary views, but their active practice has strangled it. 

 I The first step towards emancipation was the recognition of 



I sporangia as parts sni generis. Eichler, agreeing with Braun 

 and Strasbiirger, found it "highly probable according to the 

 I theory of descent " that such a structure as the ovule has uni- 

 j versally the same morphological dignity. It remained for { loebel 

 . to make the general statement that sporangia stand in a cate- 

 .' gory by themselves, and are probably not the result of modi- 

 i fication of any vegetative part. It was in this way that the 

 " phylogeneiic factor was first asserted as bearing on a question of 

 ' importance in the morphology of plants. Adherents of descent 

 j no longer passively accepted the direct results of investigation ; 

 j they began actively to check and control the interpretation of 

 j them ; but this position was not attained till more than twenty 

 J years after the publication of Darwin's "Origin of Species." 

 I Since then, however, views as to descent have taken an in- 

 ,' creasingly important place in the province of morphology, till 

 '• at the present moment a far-reaching comparison of allied forms, 

 I assisted by experiment, is the most potent instrument in the 



I hands of the morphologist. 

 But various writers admit in varying degree this factor of 

 comparison as controlling other considerations. There is indeed 

 i a wide range of difference on this point. I will cite only two 

 > extreme views. On the one hand is the view of Strasburger, 

 j which he enunciated so early as 1872. The enthusiasm for 

 evolution in the Jena school found its botanical expression in 



NO. 1 516, VOL. 59] 



the aphorism, " The highest problem of morphology is to ex- 

 plain the form of plants, but this problem can only be solved 

 genealogically." This statement is repeated in a more definite 

 form in Strasourger's text-book : " Phylogeny is thus the only 

 real basis for morphology." 



At the other extreme is the method of physiological organ- 

 ography put forward by Sachs in his Lectures. lam aware that 

 he subsequently modified his views; I merely quote the system 

 which he propounded in 18S2, as being the antithesis to that of 

 Strasburger. For in the physiological organography descent is 

 hardly taken into account at all ; parts which are plainly of 

 distinct origin by descent are classed together. This organ- 

 ography of Sachs, though introduced with all its author's charm 

 of style, never convinced the botanical world, for it treated 

 plants too much as the creatures of present circumstance. It 

 may be taken as illustrating the extreme reactionary swing of 

 the pendulum from the non- physiological attitude of the formal 

 morphologists ; a protest against the exclusion of function from 

 the morphological arena. The protest was salutary, but its 

 form was extravagant. 



Let us now consider whither "phylogeny, as the only real 

 basis of morphology," may lead us. Let us take as our pro- 

 visional view that homology in the strictest sense implies 

 repetition of individual parts, in successive generations, just as 

 the hand of the child repeats in position and qualities the hand 

 of the mother. Though among seed-bearing plants, for 

 instance, this repetition may apply for'the plant-body as a 

 whole, it will be at once apparent that such repetition as 

 regards the individual is found in comparativel.y few cases in 

 plants. The continued embryology of all the higher forms, the 

 indefinite number of the parts successively produced, and the 

 variety in detail of their arrangement show that in the strictest 

 sense repetition of individual parts cannot be traced. In a pan 

 of seedlings of the Sunflower, raised from seed of the same 

 parent, the cotyledons in all cases may be regarded as homo- 

 logous in the strictest sense, as they correspond in origin, 

 number, position, and form to like parts in the parent. In a 

 similar way the first root of the seedling appears to be indi- 

 vidually identical with the first root of the parent, or of any 

 other seedling of the batch. In those plants in which a foot or 

 suspensor is present occupying a constant position with regard 

 to the parts of the embryo, it will not be doubted that within 

 near lines of affinity the foot in any one specimen corresponds 

 to that of any other. The exact repetition which is thus 

 found to exist may be regarded as the most complete type of 

 homology. 



Starting from this repetition of individual parts in plants 

 nearly related, there is a divergence by gradual steps in two 

 directions : Firstly, in the individual plant, where the later 

 formed parts may assume forms and positions which may even 

 raise a question of their essential correspondence. Thus in the 

 batch of Sunflower seedlings there may be a varying number 

 of leaves, with varying transition from the decussate to the 

 alternate arrangement, intervening between the cotyledons and 

 the capitulum. As they vary in number and position these 

 cannot in the strictest sense be accepted as individually com- 

 parable, each to each by descent — the lineal representatives of 

 like individual parts in the parent. The lateral roots also, 

 though all essentially similar, do not correspond each to each, 

 either in number or in position. 



Again, to go a step further, a Fern prothallus produces 

 antheridia and archegonia ; their number and position are not 

 uniform ; by conditions of culture we have them under control, 

 and can induce antheridia only, or we can induce a formation 

 of archegonia upon the upper surface, where they are usually 

 absent. Plainly these cannot be held severally as the exact 

 representatives of like individual parts in a previous generation. 

 Another exceptional, but most interesting, case is that of 

 .Aspidium auomalum^ Hk. and Arn., which Sir W'illiam 

 Hooker remarks is possibly an abnormal form of Aspidiuin 

 (Polyst.) ttculeatum, Sw. In this Fern the sori, instead of 

 being all on the lower surface, as in allied Ferns, are often upon 

 the upper surface of the leaf There is no sign of torsion to 

 explain the anomaly, while the sori themselves present no 

 structural peculiarity except that they are sometimes quite 

 destitute of indusium. There has doubtless been a transfer of 

 developmental capability from the usual position of the sori to 

 the anomalous one. In case of such transfers as these we do 

 not doubt that the parts in question are to be ranked as com- 

 parable to those in the normal position ; we contemplate here, 



