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ANGIOSPERMS
larger groups has given us different systems which, if they all lay0 claim to associating in some measure natural groups, vary in the degree to which convenience or genetic relation—real or imagined—has influenced them. In Great Britain the system adopted by Bentham and Hooker—itself framed upon that of De Candolle, which in turn owed its origin to Jussieu—holds sway, with some modification, fortified as it is by the detailed description in their magnificent work—the Genera Plantarum— Taxonomy, can as . yet say little Unti! weH on in the of all genera known at the time of writing. It is :— Dicotyledones. Mesozoic period geological history tells us f Thalamiflorce. nothing about Angiosperms, and then only by their PoLYPETALiE-j Discifiorce. vegetative organs. We readily recognize in them now[ Calycifiorce. a-days the natural classes of Dicotyledones. and Mono{Inferoe. cotyledones distinguished alike in vegetative and in Heteromera;. Bicarpellatcc. reproductive construction, yet showing remarkable MONOCHLAMYDEiE. parallel sequences in development; and we see that the Gymnospermas. Dicotyledones are the more advanced and show the greater Monocotyledones. capacity for further progressive evolution. But there is The inclusion of Gymnosperms amongst Dicotyledones was no sound basis for the assumption that the Dicotyledones are derived from Monocotyledones; indeed, the palaeonto- more a conforming with conventional practice than the expression an opinion of genetic relationships which were recognized as logical evidence seems to point to the Dicotyledones being of cryptogamic. Apart from this, the grouping in the Polypetalaj the older. This, however, does not entitle us to assume and Gamopetake indicates in the main the lines of progression the origin of Monocotyledones from Dicotyledones, although of Angiosperms ; the juxtaposition of Inferse and Calyciflors; adopted to bring into prominence the similarities of facies there is manifestly a temptation to connect helobic forms being between the flower-features of the highest Polypetahe and the of the former with ranal ones of the latter. There is no aggregate Gamopetalee. The Monochlamydese, on the other doubt that the phylum of Angiosperms has not sprung hand, was retained as a convenient group for apetalous and often unisexual families, the relationships of many of whfch were and from that of Gymnosperms. are doubtful, although the connexion of some of them with Within each class the flower - characters as the essential polypetalous families was clearly pointed out. In Germany feature of Angiosperms supply the clue to phylogeny, but the there has recently been completed Die Naturlichen Pflanzenuncertainty regarding the construction of the primitive angio- familien—a book made possible by Bentham’s and Hooker’s spermous flower gives a fundamental point ol divergence m labours — the combined work of many botanists under the attempts to construct progressive sequences of the families. editorial guidance of Engler and Prantl. It gives brief diagnoses Simplicity of flower-structure has appeared to some to be always of all known genera. The system adopted in it is primitive, whilst by others it has been taken to be always Dicotyledones. derived. There is, however, abundant evidence that it may have Archichlamyde® {Choripetalce and Apetalce). the one or the other character in different cases. Apart from Metachlamydea; {Sympetalce). this, botanists are generally agreed that the concrescence of parts Monocotyledones. of the flower-whorls—in the gynseceum as the seed-covering, and in the corolla as the seat of attraction, more than in the In this system the Apetalse disappear as a group, and the androecium and the calyx—is an indication of advance, as is also families are assigned their positions in the several phyla or the concrescence that gives the condition of epigyny. Dorsi- series, as they are termed, to which in the views of the writers ventrality is also clearly derived from radial construction, and they belong. The series are arranged in genetic sequence, so anatropy of the ovule has followed atropy. We should expect far as a linear arrangement permits of this, beginning with the albuminous state of the seed to be an antecedent one to the Casuarineae and Piperacese which are regarded as the most exalbuminous condition, and the recent discoveries in fertiliza- primitive forms extant. As an attempt at a phylogenetic tion tend to confirm this view. Amongst Dicotyledones the arrangement this system is now preferred by many botanists, gamopetalous forms are admitted to be the highest develop- particularly in America. More recently a startling novelty in ment and a dominant one of our epoch. Advance has been along the way of system has been produced by Van Tieghem, as two lines, markedly in relation to insect-pollination, one of follows :— Monocotyledones. which has culminated in the hypogynous epipetalous bicarpellate forms with dorsiventral often large and loosely arranged Liorhizal Dicotyledones. flowers, and the other in the epigynous bicarpellate smallDicotyledones. flowered families of the Aggregate. In the polypetalous forms Insemineas. progression from hypogyny to epigyny is generally recognized, Seminejs. JJnitegmineae. and where dorsiventrality with insect - pollination has been Bitegminecc. established, a dominant group has been developed as in the Leguminosse. The starting-point of the class, however, and the position The most remarkable feature here is the class of Liorhizal within it of apetalous families with frequently unisexual flowers have provoked endless discussion, and there is no uniformity in Dicotyledones, which includes only the families of Nymphseacese opinion upon these matters. In Monocotyledones a similar advance and Graminese. It is based upon the fact that the histological from hypogyny to epigyny is observed both in a geo-aerophilous differentiation of the epidermis of their root is monocotyledonous, and in a hydrophilous series, but the class does not appear to be whilst they have two cotyledons—the old view of the epiblast now progressive. In this connexion it is noteworthy that so as a second cotyledon in Graminese being adopted. But recent inmany of the higher forms are adapted as bulbous geophytes, or vestigation shows that the embryo of Nymphseacese is monocotyleas aerophytes to special xerophilous conditions. The Graminese donous, and the adult root-character is only confirmation of this, offer a prominent example of a dominant self-pollinated or wind- and whatever the epiblast be it is not a constant feature of pollinated family, and this may find explanation in a multiplicity grass-embryos. Ovular characters determine the grouping in the Dicotyledones, Van Tieghem supporting the view that the of factors. From an early period in the century systematists have been integument, the outer if there be two, is the lamina of a leaf of endeavouring to group the families of the two classes of Angio- which the funicle is the petiole, whilst the nucellus is an outsperms in larger associations under such designations as Alliance, growth of this leaf, and the inner integument, if present, an Cohort, Series, and such endeavours were in the direction of indusium. The Inseminese include forms in which the nucellus discovering real phyla—although in the pre-Darwinian period is not developed, and therefore there can be no seed. The plants the idea was not always present. The relationship of some included are, however, mainly well-established parasites, and the families is so clear that there has never been any doubt about absence of nucellus is only one of those characters of reduction their forming phyletic groups, but on the other hand there are to which parasites are liable. Even if we admit Van Tieghem s many, either isolated or of reduced type, the position of which interpretation of the integuments to be correct, the diagnostic has been and in very many cases is still open to discussion, and mark of his unitegminous and bitegminous groups is simply that consequently the limits of these phyla—like the limits of families, of the absence or presence of an indusium, not a character of genera, and species—have been variously assigned by different great value elsewhere, and, as we know, the number of the botanists according to their particular views and to the knowledge ovular coats is inconstant within the same family. At the same of their time. The arrangement by botanists of these phyla in time the groups based upon the integuments are of much the
plant-structures, a view for which there is, however, no sufficient evidence. The position of Angiosperms as the highest plant-group is unassailable, but of the point or points of their origin from the general stemp of f the plant kingdom, Phytogeny and of the I)ath or pat s G their evolution, we
