As a step towards such hypothesis it has been noted that the Antarctic, the South African, and the Australian floras have many types in common.
To quote further: Existing floras exhibit only one moment in the history of the earths vegetation.
The remark may conveniently find its place here that plants which have reached a high degree of adaptive specialization have come to the end of their tether: a too complicated adjustment has deprived them of the elasticity which would enable them to adapt themselves to any further change in their surroundings, and they would pass away with conditions with which they are too inextricably bound up. Vast floras have doubtless thus found their grave in geologic change.
As Clement Reid remarked: World-wide floras, such as seem to characterize some of the older periods, have ceased to be, and plants are distributed more markedly according to geographical provinces and in climatic zones.
Starkie Gardner has argued with much plausibility that the Tertiary floras which have been found in the far north must have been of Eocene age.
If mountains serve as barriers which arrest the migration of the vegetation at their base, their upper levels and summits afford lines of communication by which the floras of colder regions In the northern hemisphere can obtain a southern extension even across the tropics.
Thus comparing the Nearctic and Palaearctic floras we find striking differences overlying the points of agreement already indicated.
A detailed examination of mountain floras shows that a large local element is present in each besides the arctic. The one is in tact the result of similar physical conditions to that which has produced the other.
At the same time the then existing alpine floras descended to lower levels, though we may agree with Ball that they did not necessarily become extinct at higher ones as long as any land-surface remained uncovered by ice.
At the close of the glacial period the alpine floras retreated to the mountains accompanied by an arctic contingent, though doubtless many species of the latter, such as Salix polaris, failed to establish themselves.
High mountain levels supplied paths of communication for stocking the South Temperate region, the floras of which were enriched by adapted forms of tropical types.
This has brought about superficial resemblance in the floras of different countries.
Oceanic islands have, as a rule, distinctive faunas and floras which resemble, but are not identical with, those of other islands in similar positions.
Their flora is far closer akin to the floras of N.
In central Russia the species become still more numerous, and, though the local floras are not yet complete, they number 850 to 1050 species in the separate governments, and about 1600 in the best explored parts of the S.W.
The central region is a transition ground where these floras find representation generally in deteriorated and dwarfed species.
The question of the mode in which the floras of islands and of continents have been formed gave rise to important speculations by such eminent botanical travellers as Charles Darwin, Sir J.
Hooker, A Lecture on Insular Floras (London, 1868); E.
As for the explanation of the community between the alpine and arctic floras, all authorities are agreed that the key to the problem is furnished by the occurrence of the glacial period.
In the ice-free belt, between the northern ice-sheet and the vastly extendedglaciers of the Alps, the two floras must have found a common refuge and congenial conditions of existence; and this view is confirmed by direct palaeontological evidence.
In many parts of the world there is no sharp line of demarcation between the Devonian and the Carboniferous rocks; neither can the fossil faunas and floras be clearly separated at any well-defined line; this is true in Britain, Belgium, Russia, Westphalia and parts of North America.
The groups of organisms utilized for zoning and correlation by different workers include brachiopods, pelecypods, cephalopods, corals, fishes and plants; and the results of the comparison of the faunas and floras of different areas where Carboniferous rocks occur are generalized in the table below.
But even within the limits of the Lower Gondwana series there are great diversities of vegetation, three distinct floras occurring in the three great divisions of that formation.
Although during the Gondwana period the flora of India differed greatly from that of Europe, it was strikingly similar to the contemporaneous floras of South America, South Africa and Australia.
Its vegetation is in point of fact of a composite character, and is constituted by the meeting and more or less blending of adjoining floras, - those of Persia and the south-eastern Mediterranean area to the north-west, of Siberia to the north, of China to the east, and of Malaya to the south-east.
At the two ends of Cajon Pass, only four or five kilometres apart, are the two utterly distinct floras of the Mohave desert and the San Bernardino valley.
He was distinguished for his researches on the Tertiary floras of various parts of Europe, and on the fossil floras of Australia and New Zealand.
The separation of the coast and interior floras is almost complete; only along the mountain passes and river valleys, and rarely there, is there an exchange of species.
Leguminosae, the two most numerous orders of phanerogams, but in number of individual plants it probably far exceeds either; whilst from the wide extension of many of its species, the proportion of Gramineae to other orders in the various floras of the world is much higher than its number of species would lead one to expect.
See also accounts of the family in the various great floras, such as Ascherson and Graebner, Synopsis der mitteleuropdischen Flora; N.
There is, however, abundant evidence that the Ferns were represented in the most ancient floras known, though they were not such a dominant group as has hitherto been supposed.
There is no doubt that the Cycadophyta, using the term suggested by Nathorst in 1902, was represented in the Mesozoic period by several distinct families or classes which played a dominant part in the floras of the world before the advent of the Angiosperms. In addition to the bisporangiate reproductive shoots of Bennettites, distinguished by many important features from the flowers of recent Cycads, a few specimens of flowers have been discovered exhibiting a much closer resemblance to those of existing Cycads, e.g.
In Rhaetic, Jurassic and Wealden floras, the Ginkgoales were exceedingly abundant (Map B, G i -G 17); in addition to A, Ginkgodium, Japan (Jurassic).
There are a few points suggested by a general survey of the Mesozoic floras, which may be briefly touched on in conclusion.
In following the progress of plant-life through those periods in the history of the earth of which records are left in ancient sediments, seams of coal or old land-surfaces, we recognize at certain stages a want of continuity between the floras of successive ages.
It is in rocks of Upper Triassic and Rhaetic age that abundant remains of rich floras are met with, and an examination of the general features of the vegetation reveals a striking contrast between the Lower Mesozoic plants and those of the Palaeozoic period.
This description applies almost equally to the floras of the succeeding Jurassic and Wealden periods.
Unfortunately, our knowledge of the later floras in the southern hemisphere is very incomplete, but a similar transformation appears to have characterized the vegetation south of the equator.
At a later stage - in postWealden days - it was the appearance of Angiosperms, probably in northern latitudes, that formed the chief motive power in accelerating the transition in the fades of plant-life from that which marked what we have called the Mesozoic floras, to the vegetation of the Upper Cretaceous and Tertiary periods.
From the floras of the Tertiary age we pass by gradual stages to those which characterize the present phase of evolutionary progress.
Among modern floras we find here and there isolated types, such as Ginkgo, Sequoia, Matonia, Dipteris and the Cycads, persisting as more successful survivals which have held their own through the course of ages; these plants remain as vestiges from a remote past, and as links connecting the vegetation of to-day with that of the Mesozoic era.
Ward, " Status of the Mesozoic Floras of the United States," Twentieth Ann.