536 



NATURE 



[OcTOi3i:r< I, 1896 



results wWch are only confirmeil by the sliuly of the reproductive 

 organs. 



In this matter fossil botany is likely to react in a beneficial 

 way on the study of recent plants, calling; attention to points of 

 strucldre which have been passed over, and showing us the value 

 of cliaracters of a kind to which systematists had until recently 

 ]«!<) I)ul little attention. At present, owing to the work of 

 1-La.llkiifer, Vesque, and others, anatomical characters are 

 ijr.ulually coming into use in the classification of the higher 

 plants, and in some quarters there may even be a tendency to 

 over-estimate their importance. Such exaggeration, however, is 

 only a temporary fault incident to the introduction of a com- 

 paratively new method. In the long run nothing but good can 

 result from the eftbrt to place our classification on a broader 

 basis. In most cases the employment of additional characters 

 will doubtless serve only to further confirm the affinities already 

 detected by the acumen of the older taxonomists. There are 

 plenty of doubtful points, however, where new light is much 

 needed ; and even where the classification is not affected it will 

 be a great scientific gain to know that its divisions are based on 

 a comparison of the whole structure, and not merely on that of 

 particular organs. 



The fact that anatomical characters are adaptive is undeniable, 

 but this applies to all characters, such difference as there is 

 being merely one of degree. Cases are not wanting where the 

 vegetative tissues show greater constancy than the organs of 

 reproduction, as, for example, in the Marattiacea;, where there is 

 a great uniformity in anatomical .structure throughout the family, 

 while the sporangia show the important differences on which 

 the distinction of the genera is based. It is in fact a mistake to 

 suppose that anatomical characters are necessarily the expression 

 of recent adaptations. On the contrary, it is easy to cite 

 examples of marked anatomical peculiarities which have become 

 the common property of large groups of plants. 



For instance, to take a case in which I happen to have been 

 specially interested, the presence of bast to the inside as well as 

 to the outside of the woody zone is a modification of dicotyledon- 

 ous structure which is in many groups, at least of ordinal value. 

 The peculiarity is constant throughout the orders Onagracea;, 

 Lythraceis, Myrtacea:, Solanacecc, Asclepiadacefc, and Apocy- 

 nacefe, not to mention some less important groups. In other 

 families, such as the Cucurbitacere and the Gentianea;, it is 

 nearly constant throughout the order, but subject to some 

 exceptions. Among the Composite a similar, if not identical, 

 peculiarity appears in some of the sub-order Cichoriaceae, but is 

 here not of more than generic value. In Campanula the systematic 

 importance of internal phloem is even less, for it appears in 

 some species and not in others. Lastly, there are cases in 

 which a similar character actually appears as an individual 

 variation, as in Carum Carz'i, and, under abnormal conditions, 

 in Phaseohis niultiflorus. 



These latter cases seem to me worthy of special study, for in 

 them we can trace, under our very eyes, the first rise of 

 anatomical characters which have elsewhere become of high 

 taxonomic importance. A comparative study of the anatomy of 

 any group of British plants, taking the same species growing 

 under different conditions, would be sure to yield interesting 

 results if any one had the patience to undertake it. 



Enough has been said to show that a given anatomical char- 

 acter may be of a high degree of constancy in one group while 

 extremely variable in another, a fact which is already perfectly 

 familiar as regards the ordinary morphological characters. For 

 example, nothing is more important in phanerogamic classifica- 

 tion than the arrangement of the floral organs as shown in 

 ground-plan or floral diagram. \'et Prof. Trail's observa- 

 tions, which he has been good enough to communicate to me, 

 show that in one and the same species, or even individual, of 

 Poly^oiiuui, almost every conceivable variation of the floral 

 diagram may be found. 



There is, in fact, no "royal road" to the estimation of the 

 relative importance of characters ; the same character which is 

 of the greatest value in one group may be trivial in another ; 

 and this holds good equally whether the character be drawn 

 from the external morphology or from the internal structure. 



Our knowledge of the comparative anatomy of plants, from 

 this point of view, is still very backward, and it is quite possible 

 that the introduction of such characters into the ordinary work 

 of the Herbarium may be premature : certainly it must be con- 

 ducted with the greatest judgment and caution. We have not 

 yet got our data, but every encouragement should be given to 



the collection of such data, so that our classification in the 

 future may rest on the broad foundation of a comparison of the 

 entire structure of plants. 



In estimating the relative importance of characters of different 

 kinds we must not forget that characters are often most constant 

 when most adaptive. Thus, as I'rof. Trail informs me, the 

 immense variability of the flowers of Po/y^'oiiiiw goes together 

 with their simple method of self-fertilisation. The exact 

 arrangement is of little importance to the plant, and so variation 

 goes on unchecked. In flowers with accurate adaptation to 

 fertilisation by insects such variability is not found, for any 

 change which would disturb the perfection of the mechanism is 

 at once eliminated by natural selection. 



MlSTOI.OC.Y. 

 I propose to say but little on questions of minute histology, a 

 subject which lies on the borderland between morphology and 

 physiology, and which will be dealt with next Tuesday far more 

 competently than I could hope to treat it. Last year my 

 predecessor in the presidency of this Section spoke of a 

 histological discovery (that of the nucleus, by Robert Brown) as 

 " the most epoch-making of events" in the modern history of 

 botany. The histological questions before us at the present day 

 may be of no less importance, but we cannot as yet see them in 

 proper perspective. The centrosomes, tho.se mysterious proto- 

 plasmic particles which have been supposed lu preside over the 

 division of the nucleus, and thus to determine the plane of 

 segmentation, if really permanent organs of the cell, would 

 have to rank as co-equal with the nucleus itself. If, on the 

 other hand, as some think, they are not constant morphological 

 entities, but at most temporary structures differentiated ad hoc, 

 then we are brought face to face with the question whether the 

 causes of nuclear division lie in the nucleus itself or in the 

 surrounding protoplasm. 



Nothing can be more fascinating than such problems, and 

 nothing more difficult. We have, at any rate, reason to con- 

 gratulate ourselves that Engli.sh botanists are no longer 

 neglecting the study of the nucleus and its relation to the cell. 

 For a long time little was done in these subjects in our country, 

 or at least little was published, and botanists were generally 

 content to take their information from abroad, not going beyond 

 a mere verification of other men's results. Now w-e have 

 changed all that, as the communications to this Section 

 sufticiently testify. 



Nothing is more remarkable in histolog)' than the detailed 

 agreement in the structure and behaviour of the nucleus in the 

 higher plants and the higher animals, an agreement which is 

 conspicuously manife.st in those special divisions which take 

 place during the maturation of the sexual cells. Is this striking 

 agreement the product of inheritance from common ancestors, 

 or is the parallelism dependent solely on similar physical 

 conditions in the cells ? This is one of the great questions upon 

 which we may hope for new light from the histological discussion 

 next week. 



Alternation ok Gener.viions. 

 W'e have known ever since the great discoveries of Ilofmeister 

 that the development of a large part of the vegetable kingdom 

 involves a regular alternation of two distinct generations, the 

 one, which is sexual, being constantly succeeded — so far as the 

 normal cycle is concerned — by the other which is asexual. This 

 alternation is most marked in the mosses and ferns, taking these 

 words in their widest sense, as used by Prof. Campbell in his 

 recent excellent book. In the Bryophyta, the ordinary moss or 

 liverwort plant is the sexual generation, producing the ovum, 

 which, when fertilised, gives ri.se to the moss-fruit, which here 

 alone represents the asexual stage. The latter forms spores 

 from which the sexual plant is again developed. 



In the Pteridophyta the alternation is equally regular, but the 

 relative development of the two generations is totally different, 

 the sexual form being the insignificant prothallus, while the 

 whole fern-plant, as we ordinarily know it, is the asexual 

 generation. 



The thallus of some of the lower Br)'ophyta is quite comparable 

 with the ])rothallus of a fern, .so as regards the sexual generation 

 there is no difficulty in seeing the relation of the two classes ; 

 but when we come to the asexual generation or sporophyte the 

 case is totally difl'erent. There is no appreciable resemblance 

 between the fruit of any of the Bryophyta and the plant of any 

 vascular Cryptogam. 



There is thus a great gap within the Archegoniatas : there is 



NO. 1405, VOL. 54] 



