October 3, 1901J 



NATURE 



559 



In some ovules where no vascular system appears in the integu- 

 ment, the chalazal haustorium is prominent, and it can therefore 

 at once tap the main water-supply of the ovule. We l-cnow also 

 of cellular ingrowths proceeding from the vicinity of the vascular 

 system of the raphe to the interior of the embryo-sac, and these, 

 ton, may have a conducting function. All these point toa water 

 and nutritive function in the integuments. The protective func- 

 tion of the tegumentary system to which attention has been 

 chiefly directed must be primarily only slight. It only becomes 

 prominent as the seed is formed, and then changes consonant 

 therewith, and with its changed function, proceed within it. Nor 

 can we now, with our increased knowledge of the ways in which 

 the poUen-tube may reach the embryo-sac, consider the function 

 of the integuments in forming the micropylar canal as one of so 

 much importance to the reproductive act as was formerly sup- 

 posed. We obtain, I think, a better conception of the ovule 

 in the view that the primary function of the tegumentary system 

 is that of a water-jacket and food store, and that it has been 

 developed in response to the special demands for water involved 

 in the seed-habit.' 



To the question why there are two integuments in some cases 

 and only one in others we can only reply that our knowledge of 

 ovular structure and changes is yet too slight to permit of a 

 definite opinion being expressed. We find that there is a remark- 

 able concurrence of the unitegminous ovule with a gamopetalous 

 corolla in the flower, for the character apparently holds for the 

 whole of the gamopetalous Dicotyledones excepting Primulales. 

 On the other hand, not all Polypetalx have bitegminous ovules, 

 whilst bitegmeny is usual in Monocotyledones. Recently the 

 character has been used by Van Tiegheni as one of prominence 

 in his new classification of the families of Dicotyledones. But 

 it is not so constant an one as his groups of UnitegmineK and 

 Bitegminea; would lead one to suppose. The degree in which 

 it is inconstant we cannot yet fix, because we know details of so 

 few genera. We do know, however, that all genera in one 

 family are not always alike in respect of it. In Ranunculace^, 

 for instance, the most of ihe genera with radial flowers are 

 unitegminous, whilst those with dorsiventral flowers are 

 bitegminous. Again, in Rosacere, the Potentills are uniteg- 

 minous, as is Rosa, whilst Pome?e and Prunes; are bitegminous ; 

 and of the Spirxete, Neillia is unitegminous, but the closely 

 allied Spiraea is bitegminous.- In other cases the char- 

 acter confirms distinctions ; as, for instance, in separating the 

 unitegminous Betule;ie and Coryle.Te from the bitegminous 

 Quercineoe. The explanation of all these constructions may, I 

 suggest, be sought for with better prospect of success in the 

 water-relationship and food-relationship of the integuments to 

 the embryo than in protective function and relations to pollina- 

 tion. It is, perhaps, not without significance from this point of 

 view that in, for instance, the Gamopetalre such protective 

 function as attaches to the tegumentary system in the seed is 

 reduced or extinguished through the development of indehiscent 

 fruits, accompanied in many Aggregatoe and higher Heteromera; 

 by the sinking of the gyn;vceum in the torus, and in many 

 BicarpelletiE by its enclosure in a persistent accrescent calyx. 



All the information at our disposal seems to indicate that the 

 tegumentary system of the ovule is extremely adaptive, and that 

 its characters are not of themselves of much phyletic import. An 

 extended examination of its characters as an organ of the nature I 

 have depicted in relation to embryogeny is greatly needed. It is 

 made all the more interesting by the questions of development 

 of endosperm opened by the discovery of "double fertilisation." 

 There is no more promising field of investigation than this, for 

 it must yield results infinitely more interesting than the techni- 

 calities of formal morphologj- which have been for too long the 

 stimulus to ovular research. I am tempted to go further and to 

 say that it might supply an explanation of that most puzzling of 

 subjects, the forms and curvature of the ovule. The common 

 assumption that these have relation to pollination and make the 

 advent of the pollen-tube at the micropyle easier is not alto- 

 gether satisfactory. For the curvature not infrequently seems to 

 place the micropyle in a position the opposite of favourable, and 



1 To discuss the morphological interpretations of the funicle and integu- 

 ment that have been advanced would carry me beyond the scope of this 

 address. I do not know that an axial hypothesis for any part of the ovule 

 is now maintained. The foliar interpretation of the funicle and integuments 

 as against their sporangial nature is supported by two distinct schools of 

 botanists. One approaches the subject from the standpoint of the 

 anaphytose of the earlier years of last century, and appeals largely to 

 teratology ; the other from that of vascular anatomy. I do not accept the 

 starting-point of either the one or the other. 



- Spiraea is, however, exalLuminous, whilst Neillia is albuminous. 



NO. 1666, VOL. 64] 



there is an absence of curvature in cases where it would appear 

 to be desirable. 



I will not dwell upon the subject of the seed itself as an 

 advantage to the Angiosperm. Its construction follows upon 

 the successful water-relation previously secured. We all know 

 how its manifold adaptations to dissemination bring about its 

 fortuitous deposition upon various soils, and the embryo is 

 placed well guarded within the seed-coat ready to take advan- 

 tage of the moment when moisture is sufficient for its 

 germination. 



Whilst the seed-habit is the character which has primarily 

 given to Angiosperms their advantage as a land-type,' their 

 vegetative organs also show an advance in their relationship to 

 water upon those of the forms they have supplanted. I have 

 already remarked that the growth-forms of the vegetation of the 

 present day are the same as those of old. That means that the 

 early as well as the later groups of vegetation have solved in 

 much the same way, so far as general form is concerned, the 

 problem of the exposure in the atmosphere of a large assimilating 

 area with a sufficient mechanical support and adequate water- 

 supply. That wherever a water-carrying system is found in these 

 growth-forms it dominates the anatomy is witness to the import- 

 ance of the water-relationships I wish to emphasise. 



There are two features in the water-carrying system of Angio- 

 sperms in which they are superior to the older types — namely, 

 their general monostely and their vasa. 



No one will contest that polystely is a less perfect mechanism 

 for water-carriage in a massive plant than is monostely. The 

 limitation imposed by it to an increment in the area of carriage 

 contrasts unfavourably with the openness in this respect possessed 

 by monostely. In the moister climatic conditions of the age or 

 domination of Pteridophytes polystely may have well sufficed 

 for the water-needs of the plants, especially of the dwarfer 

 forms ; but even then, as we know, monostely was the habit in 

 many of the larger tree-forms, and the development of a 

 cambium enabled them to provide for continued additions to 

 their carrying system. Where such monostely and secondary 

 growth occurred in these older types their adaptation in these 

 respects to water-carriage was on lines similar to those of our 

 dominant Dicotyledones and was effective in giving them 

 dominance in their epoch. There is no more interesting page 

 in the history of evolution than that — and we owe it in large 

 measure to the labours of Scott and Seward — upon which is 

 depicted the struggle of some polystelic forms amongst these old 

 plants to achieve the structural facilities more easily attained 

 through monostelic construction. The existence of polystely in 

 a few Angiosperms only confirms the advantage which the whole 

 group has derived from its monostely. Such polystelic forms 

 amongst them as we know have many of them special water- 

 adaptations, and in no case can they be said to be progressive 

 types. 



I do not need to remind you that vasa are not the exclusive 

 possession of the angiospermous type, but they are the con- 

 spicuous feature of their carrying system, whilst the tracheid is 

 the leading one in the older type of vegetation. All anatomical 

 evidence indicates that vasa give greater facility to rapid trans- 

 port of water than do other elements, and we may, therefore, 

 conclude that they have been adjuvants in enabling the Angio- 

 sperm to meet efiectively the demand made upon it by the drier 

 atmospheric conditions. 



I now pass on to consider from the same standpoint the 

 classes which make up the group of Angiosperms. 

 Of the Classes of Angiosperms. 



There has been for long a general recognition of two classes 

 amongst the Angiosperms — Dicotyledones and Monocotyledones 

 — separated one from the other by definite characters which I 

 need not specially depict here. Recently, however, we have 

 seen an attempt made by Van Tiegheni to establish another 

 class — that of Liorhizal Dicotyledones — for which is claimed a 

 rank equal to that of the Dicotyledones and Monocotyledones. 

 Were this valid it would be a matter of supreme importance, for 

 whatever be the relationship between Dicotyledones and Mono- 

 cotyledones there can be no doubt of their having developed as 



1 Gymnosperms, sharing with Angiosperms the seed-habit, have in that 

 had advantage over Pteridophytes. But their flower-mechanism is much 

 less perfect. The reasons for their being bested as a class by Angiosperms 

 must be complex. Gymnosperms, as a whole as we know them, are less 

 adaptive than .\ngiosperms. The decadence of the cycadean line of descent 

 may have been helped by their conservatism in the methods of water- 

 carriage in the vegetative organs. The coniferous type has held its own in 

 the Northern Hemisphere. 



