56o 



NA TURE 



[October 3, 1901 



distinct groups within the whole period of which we have 

 knowledge of them, and the existence of a third class inter- 

 mediate or outside of them might lead lo interesting conclusions. 

 It is worth while, therefore, to consider the evidence on which 

 this class is founded. It includes two of our recognised families 

 — the Nympha;aces; and the Graminea;. 



What is the exact position and the affinities of the Nym- 

 ph^acece amongst Angiosperms is no new theme of discussion. 

 That they have characters resembling those of Monocotyledones' 

 has often been insisted on. Van Tieghem lays stress on what he 

 considers the monocotylous dift'erentiation of the root-apex and 

 the derivation of the piliferous layer from the same meristem- 

 initials as the cortex, whilst in the embryo he finds the two 

 cotyledons of Dicotyledones. But the most recent observations 

 of the embryogeny of the family go to show that the embryo is 

 that of the monocotylous plants, the apparent dicotylous 

 character being the result of the splitting of one cotyledon. If 

 this be so the position of Nymphxaceae will be amongst the 

 Monocotyledones, a position the root-characters in Van Tieghem's 

 view will support. But whether this be confirmed by further 

 research or no — and a complete reinvestigation of their embry- 

 ogeny and development is much wanted — what we may say at 

 present is that it is not in features such as this one of the root- 

 apex — which is, after all, not so simple and uniform as Van 

 Tieghem would have it — that we are likely to find phyletic 

 diagnostic characters of groups. 



The reason for the inclusion of the Gramineje in this new 

 group is the assumed presence of a second cotyledon. The 

 construction of the embryo of grasses is peculiar, as is well- 

 known, and has for a long time been a main support of the 

 hypothesis that the Monocotyledones are derived from the 

 Dicotyledones ; for here alone, since the dicotylous character of 

 forms like the Dioscore^ was shown to be untenable, was there 

 a structure which could be interpreted as evidence of a reduced 

 second cotyledon. The idea that the epiblast is such a structure 

 was enunciated by Poiteau at the beginning of the last century, 

 and along with hypotheses of the nature of the other parts of 

 the grass-embryo has been a subject of vigorous discussion since 

 that lime. The controversy is not yet closed. Whilst we have 

 Van Tieghem now adopting the view of the cotylar nature of 

 the epiblast and using it as a character of fundamental taxonomic 

 importance, we have others who as strongly uphold the inter- 

 pretation of it, first formulated by Gaertner, as a winged 

 appendage of the scutellum, which is considered to be the 

 cotylar lamella. And, again, there are those who take the 

 view that it is a mere outgrowth of the hypocotylar body of the 

 embryo and without any cotylar homology. Our interpretation 

 of the part must depend primarily upon the standpoint from 

 which we view the embryo of Angiosperms. This I shall 

 discuss presently. All I need say here, iiprofos of the class of 

 Liorhizal Dicotyledones, is that whatever the epiblast be — and 

 for my part I am disposed to regard this simple cellular structure 

 as merely an outgrowth with a water-function from the embry- 

 onal corm — a dispassionate consideration must lead us to hold 

 that it is a bold step to use a character the morphological 

 value of which can be so variously interpreted as one of primary 

 importance for separation of a group of Angiosperms. More- 

 over, we must remember that the feature of the epiblast is not 

 one of universal occurrence in the Gramineje. If we take a 

 well-defined tribe like the Hordes, as framed by Bentham and 

 Hooker, we find that of eight of its twelve genera which have 

 been examined for this feature five have the epiblast and three 

 want it. And surely the fact of its presence in Triticum and 

 absence in Secale, its presence in Elymus and absence in 

 Hordeum, is strong evidence that the epiblast is not a character 

 of such importance as it would have were it a reduced cotyledon 

 as is asserted. 



It appears to me, therefore, that this third class of Angiosperms 

 has no .sound foundation, no more, perhaps less, than Dictyogens 

 and Rhizogens which appeared as parallel groups with Endogens 

 and Exogens in Lindley's old classification. Our present 

 knowledge allows the recognition of only two classes of the 

 angiospermous type — the Dicotyledones and the Monocotyle- 

 dones. 



0/ Dicotyledones and Monocotykdoucs. 



The relationship of these two groups is involved in the origin 

 of the angiospermous type. They may have had a common 



this resemblance was chiefly 



NO 1666, VOL. 64] 



origin or they may have arisen separately ; and if the former the 

 Dicotyledones may have been a subsequent offshoot from the 

 Monocotyledones, or the reverse may have been the case. Each 

 of these possibilities has its supporters. Were I to maintain an 

 opinion it would be that the two classes have arisen on separate 

 lines of descent. The embryo-characters, as well as those of the 

 epicotyl, can, I think, be shown to be fundamentally different 

 and to afford no basis for an assumed phyletic connection. The 

 differences between Hepaticre and Musci, to take a parallel case 

 in a lower grade, are not more conspicuous. The parallel 

 sequence in development in the two classes is no more than 

 one would expect, and may be regarded as homoplastic. To 

 the question which group is the older I would answer that the 

 Dicotyledones are by far the most adaptive and progressive if — 

 as is not necessarily the case — this can be taken as evidence of 

 their more recent origin. This, however, is not the matter I 

 intend to discuss here. I wish rather to inquire if there are any 

 features broadly characterising the groups to which, as in the 

 case of Angiosperms as a whole, we may look for help to an 

 explanation of the predominance at this time of the type of 

 Dicotyledones. I think there are, but they are not to be found 

 in the reproductive system. That is constructed on sufficiently 

 similar lines in each class. The features I refer to are to be 

 found in the construction of the vegetative system both in the 

 embryo and in the adult. That of the former gives the dicotylous 

 plant an advantage in its start on life ; that of the latter, both in 

 shoot-system and root-system, is better adapted in Dicotyledones 

 in relation to water-supply. 



I specially differentiate the embryo-condition from the adult 

 because in our consideration of these higher plants we are apt 

 to overlook the two distinct stages into w-hich their life is 

 divided, and which call for altogether different adaptations. 

 There is, firstly, the life in the seed and in germination ; and, 

 secondly, there is the life after germination. The conditions 

 and the manner of life are not alike in the two stages. In the 

 first the plant is heterotrophic, in tlie second it is autotrophic. 

 The functions of the portion of the plant which lives the life 

 within the seed, and which bears the incipient epicotyl and 

 primary root as small, at times hardly developed, parts, are to 

 absorb food, either before germination, as in exalbuminous 

 seeds, or during germination in albuminous seeds, to rupture 

 the seed-coat, and to place the plumular bud and the primary 

 root in a satisfactory position for their growth and subsequent 

 elongation. The functions of the adult may be summarised as 

 the development and maintenance of a large assimilating and 

 absorbing area preparatory to reproduction. 



We ought, I think, to look upon the embryo as a proto- 

 corm ' of embryonic tissue adapted to a seed-life. Under the 

 influence of its heterotrophic nutrition and seed-environment 

 it may develop organs not represented in the adult plant as 

 we see in, for instance, the embryonal intraovular and extra- 

 ovular haustoria it often possesses. There is no reason to 

 assume that there must be homologies between the protocorm 

 and the adult outside an axial part with its polarity. There 

 may be homologous organs. But neither in ontogeny nor in 

 phylogeny is there sutiicient evidence to show that the parts of 

 the embryo are a reduction of those of the adult. - 



The protocorm has, I believe, developed along different lines 

 in the Dicotyledones and Monocotyledones. This has been to 

 the advantage of the former in the provision that has been made 

 for rapid as opposed to sluggish further development. Confining 

 ourselves to the general case, the axial portion of the protocorm 

 of the Dicotyledon, the hypocotyl, bears a pair of lateral out- 

 growths, the cotyledons, and terminates in the plumular bud 

 and in the primary root respectively. The cotyledons are its 

 suctorial organs, and the hypocotyl does the work of rupturing 

 the seed and placing the plumular bud and root by a rapid 



1 The term has already been used for the embryo of Orchidea:, where the 

 axis is tuberous as is the structure to which the term has been given_ in 

 Lycopodinese. But tuberousness is not an essential for the designation 



- I cannot pursue the subject here, nor discuss the view of the cotyledtjns 

 as either ancestral leaf-forms or arrested epicotylar leaves. The analogies 

 with existing Pieridophytes that are cited are not pertinent, for there is no 

 evidence that Angiosperms have that ancestry, cr indeed that their phylo- 

 geny was through forms with free embryos. Nor is the fact of resemblance 

 between cotyledons and epicotylar leaves and the existence of transitions 

 between them convincing. That the cotyledons, primarily suctorial organs, 

 should change their function and become leaf-like under the new conditions 

 after germination is no more peculiar than that the hypocotyl should take 

 the form of an epicotylar internode, from which it is irilrinsically different 

 as the frequent development upon it of hypocotylar buds throughout its 

 extent shows. 



