October 3, 1901] 



NA TURE 



^6i 



elongation ' which commonly brings the plumular hud above 

 ground, protected, it may be, by the cotyledons. These latter 

 may then become the first assimilating organs unlike or like to 

 the epicotylar leaves. In the Monocotyledones the axial portion 

 of the protocorm has usually no suctorial outgrowths. Its apex 

 and usually its base also are of limited growth. The plumular 

 bud is a lateral development, and the primary root often 

 an internal one. The suctorial function is performed by 

 the apex of the protocorm, termed here also the cotyledon. - 

 The rupture of the seed and the placing of the plumule along 

 with the primary root — for the axis of the corm does not elon- 

 gate between them — are the work of the base of the suctorial 

 portion of the corm. 



The whole arrangement in Monocotyledones is in marked 

 contrast with that of the Dicotyledones. Instead of the free 

 axial elongation begun in the protocorm and continued upwards 

 and downwards in the epicotyl and primary root, there is 

 limited axial growth of the protocorm with lateral outgrowth 

 of the plumular bud and arrest of the primary root. These 

 differences in the protocorm are, I think, primary, and they 

 point to independent origins of the two groups. The advantage 

 lies, as I have said, with the Dicotyledones, and we find that 

 the features of development of the protocorm are continued in 

 the adult. There is a marked contrast between the free inter- 

 nodal growth of the shoots of Dicotyledones with their copious 

 root-system and the contracted stem-growth and the arrested 

 root-system in Monocotyledones. It is interesting to note 

 further how the monocotylous type has developed so largely 

 upon restricted lines in the way of short rhizomatous, often 

 tuberous, growth, whilst the dicotylous gives us the characteristic 

 growth-form tree. 



When we compare the tree-type of the Dicotyledones with 

 that of the Monocotyledones we see at once the feature I refer to 

 in the adult, which has given the advantage to the dicotylous 

 type in respect of its water-supply. In Dicotyledones we have 

 a much-branched stem ending with numerous shoots with long 

 internodes and small apices, and bearing many small leaves 

 which are mainly deciduous. In the monocotylous tree, of 

 which we may take the palm as a type, there is a straight stem 

 with short internodes, a large apex bearing few large leaves not 

 often renewed ; if there be branching it takes more or less the 

 form of a fork. The whole of this external configuration bears 

 relationship to the internal structure. In the Dicotyledon the 

 open bundles of the central vascular system provide through 

 their cambium for a continued increase of the water-carrying 

 system and medullary rays, which, although it is to many a 

 heresy, I hold to have profound influence upon the movement 

 of water in trees. The buttressing of the branches is also 

 secured, and thus is rendered possible a large assimilating area 

 made up of a vast number of small individual surfaces, each one 

 of which can be readily thrown off. In the Monocotyledones, 

 on the other hand, the distribution of a large number of closed 

 vascular bundles in a matrix without a cambium involves the 

 provision of a broad terminal cone, gives no support, outside 

 interstitial growth, to lateral branches, which are consequently 

 when developed placed so as to give an equipose, and the 

 as.similating surface has to be concentrated in a few large leaves. 

 The possession of cambium has enabled the Dicotyledones to 

 meet in a much better way the requirements of water-supply and 

 strength in correlation with feeding. 



The general uniformity and effectiveness of the scheme of 



1 In relation to this function it is noteworthy that the hypocotyl refitively 

 seldom in the exalbuminous seed of Dicotyledones becomes the reservoir of 

 food-material, whereas in Monocotyledones the axis of the embryo is the 

 usual seat of deposition. 



2 I use the term purely as an objective designation, and in the original 

 meaning of the suctorial orgaii in the embrj'O. This terminal cotyledon in 

 the Monocotyledones is not a leaf nor the homologue of the lateral cotyle- 

 dones in the Dicotyledones. The "traceable and direct developmental 

 history in the formation "" of the two organs is clear, and they are not alike. 

 To those who hold the contrary view a terminal leaf is no obstacle. I think, 

 however, the question of lateral or terminal is of importance in organo- 

 graphy. The "sympodial leaf-from -leaf evolution," described in the first 

 ejiicotylarstages of Juncus, Pistia, and other plants, demands examination 

 with the aid of modern methods. All cases of vegetative organs in which 

 the distinctions between organs are said to break down are worthy of being 

 looked at in the light of their relation to their nutritive environment. How 

 nutrition aflFects plant-form we do not yet understand. Its effects are 

 familiar, both in vegetative and reproductive organs. The grosser cases, 

 in parasites, show in the extremes an abolition of most of the landmarks of 

 morphology — "the whole scheme of formation of organs is jumbled." 

 Heterotrophic _ "jumbles " do not, however, deny the ordinary morpho- 

 logical categories. Pseudo-terminal reproductive organs are to be expected 

 under the cessation of growth with which their development " 



NO. 1666, VOL. 64] 



cambial growth is a remarkable feature in the dicotylous type ; 

 but there is still a wide field of investigation in the relationships 

 of size and distribution of vasa both to the other structural 

 elements of the stem and to the form of the plant in relation to 

 its environment. So far as I know the monocotylous tree- 

 forms, there has been an attempt in two different directions to 

 provide an increased water-carrying system in them. There is 

 the familiar one of the secondary cortical cambium in Draca;na 

 and other genera. In them the cambium merely repeats in its 

 products the construction of the primary stem, and does not 

 provide so copious an increase of carrying area as does the 

 system in dicotylous plants. And then in such plants as Barba- 

 cenia, many Bromeliacece, perhaps Kingia, we have an airange- 

 ment reminiscent of the superficial root -system which is found 

 in many polystelic arborescent Pteridophytes of the present day. 

 There is a copious growth of adventitious roots from the central 

 vascular cylinder, and these pass down within the cortex, and 

 from its cells are no doubt able to draw water for the upper 

 parts of the stem.' Ultimately many of these roots reach the 

 soil. At best, however, neither of these systems has been satis- 

 factory. All that can be said for them is that they have enabled 

 the monocotylous trees in which they are found to hold their 

 own in xerophilous conditions. 



Of Phyla wilhin Dicotyledones and Monocolyledones. 



A brief reference only to the groups within the Dicotyledones 

 and Monocotyledones must conclude these remarks. Whilst 

 there is a wonderful concurrence in the opinion of botanists as 

 to the natural groups — real phyla, whether termed cohorts, 

 alliances, or series — into which many of the families of both 

 Dicotyledones and Monocotyledones fall, there is irreconcilable 

 divergence of view as to their genetic sequence or sequences. 

 And this is not surprising when we remember that we know 

 nothing of the starting point or points of the classes themselves : 

 and have, moreover, no critical mark by which to diagnose a 

 primitive from a reduced feature in many of the flower construc- 

 tions to which, as characteristic of Angiosperms, importance is 

 attached. The desire to establish a monophyletic sequence of 

 these phyla is natural, and finds expression in pedigrees of 

 Dicotyledones issuing from, it may be, Ranales or Piperales, of 

 Monocotyledones from, say, Apocarpiie or Arales. But all such 

 attempts appear to me, in the present state of our knowledge, to 

 be in vain. We see in the phyla, as we know them, culminat- 

 ing series in our epoch in lines of descent ; some, for instance 

 Myrtales or Lamiales, progressive ; others, like Primulales or 

 Pandales, apparently not so. We also recognise that these 

 series group themselves in many cases as branches of broader 

 lines of descent ; for example, in the BicarpellatK of Gamopetalas, 

 in the Helobiece of Monocotyledones. To a greater or less 

 degree such relationships are traceable now, and as we obtain 

 more knowledge of the angiospermous plant-life of the world 

 they will be widened. But this is a different thing from the 

 carrying back the pedigree of every phylum of dicotylous and 

 monocotylous plants to one or other of the existing ones, which 

 may possess what are taken to be elementary characters. We 

 have, so far as I know, no evidence to sanction the belief, or 

 even the expectation, that there is extant any family of Dicotyle- 

 dones or Monocotyledones which represents, even approximately, 

 a primitive type in either class. The stem in each has gone. 

 We have the twigs upon a few broken branches. 



.•\mongst the phyla we cannot discern any one type that can be 

 described as the dominant one. The multifarious adaptability 

 of the angiospermous type has given us diverse forms, suited, as 

 far as we discern, no less well to the varied environments of 

 our epoch. Vet we are able to differentiate certain of them 

 which take precedence alike in point of number of species and 

 in area of distribution. If we seek for some general character 

 that marks these advanced groups we find it in the tendency to 

 greater investiture of the ovule, both in Dicotyledones and Mono- 

 cotyledones. This is brought about in different ways ; for 

 instance, by the sinking of the gyneeceum in the torus as in 

 Compositae, by inclusion within a persistent calyx as in Labiatoe, 

 or within bracts as in Gramines;. This feature, it will be 

 observed, emphasises that which I have put in the forefront, as 

 leading to the establishment of the angiospermous type. That 

 it must give greater security to the embryo in relation to its 

 water supply is obvious, although it has evidently also direct 



1 I leave it to Palaophytologists to say whether this construction may 

 sometimes account for the profusion of roots alongside of stem-structure in 

 fossil-sections. 



