558 



NA TURE 



[October 3, 1901 



involved the concentration and the specialisation of the indi- 

 vidual. 



All this is, however, to you botanists but the commonplace 

 of your laboratories and lecture halls. But I have thought that 

 it should be said, because this fundamental difference of organi- 

 sation between the two kingdoms is apt to be forgotten in dis- 

 cussions of problems of evolution, more particularly those of 

 transmission of characters and the effect of environment. This 

 is especially so when they are approached from the zoological 

 side. Were the point always recognised we should not have 

 zoologists finding similarity between bud-variation in a flower- 

 ing plant and the change in colour of the hair of a mammal. 



Of Origin and Dominance of the Angiospernious Type. 



It is now usually admitted that all plants, like all animals, 

 have been derived from aquatic ancestors, and that the trend 

 of evolution has been in the direction of the establishment of 

 a vegetation adapted to a life on land. Of this evolution the 

 Angiosperms as we see them to-day are the highest expression. 

 Can we say anything about the origin of the angiospermous 

 type ? As the problem presents itself to me we can only mark 

 time at present. 



From the geological record we obtain no help. The earliest 

 tr aces of Angiosperms in rocks of the middle Mesozoic period 

 enable us to say little regarding them e.vcept that the fragments 

 give evidence of an organisation as complete as that possessed 

 by the Angiosperms of the present day. The gap between the 

 angiospermous and other types of vegetation is a wide one, and 

 no links are known. Until further research provides specimens 

 in a better state of preservation and showing structure we can 

 hope for little assistance from the geological record ; and when 

 we consider the circumstances in which the angiospermous 

 plants as a whole grow the prospect of such finds does not appear 

 to be very bright. 



The appeal to ontogeny likewise gives us little information. 

 ■Comparative study does not establish connection with, only dif- 

 ferentiates more and more, the types of the Pteridophytes and 

 Gymnosperms. The strong likeness of the pro-embryo after the 

 primary segmentation of many Angiosperms to the pro-embryo 

 of many Bryophytes has appeared a sufficient reason to some 

 botanists for ascribing a bryophytous parentage to the .Angio- 

 sperms. Indeed it has been said that "the monocotylous embryo 

 is the direct homologue of the sporogonium of the moss, the 

 cotyledon being homologous with the spore-producing portion of 

 this out of which it originated." This anaphytic conception of 

 the monocotylous embryo seems to nie to have as little real 

 foundation as the hypothesis of its origin. The pro-embryonic 

 resemblance is interesting, but it may as well be homoplastic as 

 genetic. 



But if the information available to us does not permit of our 

 building up a pedigree for the Angiosperms, we are on surer 

 ground when we endeavour to fix upon characters which have 

 enabled the group to become established as the dominant vege- 

 tation of our epoch. Before the era at which we have first 

 knowledge of Angiosperms the earth's surface was, we know, 

 clad with a dense vegetation composed of members of the 

 various classes of Pteridophjtes and Gymnosperms. These 

 appear to have existed in all the growth-forms which we know 

 now amongst the Angiosperms — Herb, Shrub, Tree, Liane. 

 Vet they are now represented amongst living plants by only a 

 few remanent forms. Hordes of distinct forms and whole 

 classes have disappeared, giving place to plants of the angio- 

 spermous type. There must then lie some feature or features of 

 advantage in this type over those of the groups that previously 

 occupied the ground, and through which it became dominant. 



In considering this point we must bear in mind the well- 

 known climatic differences — particularly in the distribution of 

 water — that distinguish our epoch from those in which > these 

 extinct plants throve. The factors which determine the success 

 or otherwise of an organism or group of organisms at any period 

 must always be complex, and no exception can be claimed for 

 plants in their struggle for mastery. But looking at the succes- 

 sion of plant life in the world in relation to the known diminu- 

 tion of water-surface and increase of land-area, and the conse- 

 quent differentiation of climates, we cannot but be convinced 

 that of these factors water is one which has had supreme influence 

 upon the evolution of the facies of the plant-life that we see to- 

 day. I think the statement is warranted that the Angiosperms 

 have become dominant in great measure because in their con- 

 struction the problem of the plant's relationship to water on a 



NO. 1666, VOL. 64] 



land-area has been. solved more satisfactorily than in the case ot 

 the groups that preceded them. 



The seed character — and the flower which it involves — dis- 

 tinguishes the Angiosperms. What, then, are the relationships 

 to water which the formation of seed implies and through which 

 the Angiosperm has advantage ? 



Two prominent risks in its relation to water attach to the pro- 

 cess of sexual reproduction in a plant of the type of heterosporous 

 Pteridophytes. Firstly, that of failure of moisture on the soil 

 sufficient to promote germination of the spores ; secondly, that 

 of failure of moisture on the soil sufiicient for the passage of the 

 spermatozoid to the ovum. In addition there is the risk of 

 failure of the fall of microspores and megaspores together upon 

 the soil. In the Angiosperms such risks are practically abolished 

 in the formation of flower. The stigmatic surface of the style 

 itself provides a secretion — the more copious in a dry and sunny 

 atmosphere — to moisten the pollen-grain and stimulate germina- 

 tion, and for the spontaneous movement of the spermatozoid is 

 substituted the passive carriage of the male gamete to the ovum 

 by the agency of the pollen-tub e. Possible failure of pollination 

 is, too, provided against by the complex mechanism of the 

 flower in the highest forms in relation to insect-visits. The 

 sexual act, then, might, we conceive, gradually become more 

 and more difficult of consummation to the Pteridophyte as the 

 area of dry land increased. To the seed-plant it was more 

 secure by its independence of the presence of free water. The 

 failure of performance of the function of sexual reproduction may 

 have hastened the disappearance of Pteridophytes before the 

 advance of the Angiosperms. 



But if this flower-mechanism relieves the Angiosperm from 

 risks in the performance of the sexual act, it imposes a new duty 

 upon the plant, that of nursing the embryo witliin the sporan- 

 gium. This involves a water-supply of a kind not demanded in 

 the Pteridophytes, and we may gain some idea of the import- 

 ance of this by a comparison of the trivial vascular system 

 required to supply through the stamen the pollen-grain, with 

 the copious system that traverses the gyn.'Eceum for the ovules. 

 It is, however, to the ovule — the immediate nursery of the 

 embryo — that we must look for special indications of this water- 

 relationship of which I speak. 



Perhaps no organ has given rise to more discussion than this 

 characteristic one of flowering plants. To most of us I believe 

 the controversy over its axial or foliar nature will be, in a 

 measure, historical only. All recent investigations of sporangia 

 — and to no one does Botany owe more in this respect than to 

 Bower — tend to confirm the view that it is, and always has 

 been, an organ sui generis. To that category the nucellus of the 

 ovule is now pretty generally admitted. It is the body of a 

 sporangium. But the nature of the tegumentary system and of 

 the funicle which give the ovule so distinctive a character is still 

 the subject of disagreement.' 



I do not share a view which sees in the integuments or other 

 parts of the ovule anything of an axial or of a foliar nature. 

 To me the funicle is a sporangiophore — a sporangial stalk — and 

 the tegumentary system is an outgrowth of the sporangial 

 primordiumof somewhat variable origin and development, whose 

 first function it is to carry and store water for the embryo, and 

 then also to serve as a food-reservoir. The whole construction 

 is adapted to the function claimed for it. The well-developed 

 vascular system from the placenta traverses the funicle, but the 

 subsequent fate of the nucellus forbids its passing through this, 

 and the needs in respect of water (and what it carries) of the 

 embryo and of the other further developments that proceed in 

 the embryo-sac are provided for by the production of the tegu- 

 mentary outgrowths into which the vascular system may, if 

 necessary, be continued and spread out. 



That the tegumentary covering has this function we have 

 direct proof in its penetration by haustoria, derived either from 

 the embryo itself or from the embryo-sac, which absorb from it 

 water and food for the developing embryo. These haustoria 

 appear to be much more elaborate and more widespread than 

 has been supposed, and a definite correlation has been estab- 

 lished in many cases between them and the integuments. The 

 thicker the integument the better developed is the haustorium. 



1 Scott's discovery of a bracteal investment to the inegasporangium in 

 Lepidocarpon is an interesting one in relation to the question of the enclo- 

 sure of sporangia. It shows how in the Lepidodendrea; a covering of the 

 sporangium could be developed, much in the same way as a carpellary 

 envelope in Angiosperms. Whether the ovular integument or the 

 covering in Angiosperms was the earlier development is open to di- 

 I am disposed to give precedence to the ovular coat. 



