The Australian continent, which covers 35 degrees of latitude, and has remained isolated from all other lands since the Cretaceous period, necessarily has many peculiarities of great interest in its flora. The first botanists, perhaps guided by the characteristics of the fauna, also considered the Australian flora as purely primitive and belonging to a period of geological history different from that to which the other plants that exist today on earth belong. The presence in the Australian flora of certain families of plants showing particular archaic characters, may have favored this view. Such plants are found especially among the Pteridophyta and are mainly Isoëtes and Phylloglossum (the most singular among the living Lycopodiales) and the Tmesipteris and Psilotum (the living representatives closest to the paleozoic family of the Sphenophyllales). Also among the Filicales a considerable number of ancient forms are well represented in Australia. However, although these families may be regarded as primitive, their Australian members often display curious specializations alongside archaic characters.
According to itypeusa.com, the Australian flora, as a whole, reveals a high degree of specialization, which is above all demonstrated by many interesting structural peculiarities of plants with flowers: such as the caliptra of the Eucalyptus flowers, the irritable labellum of some species of Pterostylis, the gymnostemium of the Stylidium (the trigger plant) and the xerophile modifications that will be enumerated later.
However, the Australian flora is not fundamentally different from that of other continents.
Stooker observed since 1860 that the relative proportions of Dicotyledons to Monocotyledons, of genera to orders, of species to genera, are the same as for other flora of equal extension. He also observed that the relative proportions of the Thalamiflorae, Calyciflorae and Corolliflorae they are the same as the other floras; that the proportion of Gymnosperms to Angiosperms is almost the same, and furthermore that the reactions of the Australian flora to topographical changes are analogous to those of the flora of other continents. A noteworthy feature of the Australian flora, however, is its high percentage of endemics. In comparison with any other region of equal extension, it contains the greatest number of genera and species characteristic of its area and the least number of plants belonging to other parts of the earth.
Australian plants have about 11,000 species distributed in 150 families. The 12 largest families are in the following order:
Eight families are limited only to Australia: Goodeniacee, Candolleacee, Brunoniacee, Casuarinee, Tremandracee, Stackhousiacee, Filidracee, Pittosporacee (except Pittosporum which is also found in the tropical zone of the ancient continent). Among these, the Goodeniacee, Candolleacee and Casuarinee show affinity with the Campanulaceae, Lobeliacee and Miricaceae which are widely distributed on the globe.
Andrews (p. 175; see Bibl.) Notes that perhaps the most instructive peculiarity in the distribution of the Australian flora is given by the large number of genera and species and by the innumerable individuals of these found in the vast sandy tracts, especially in the region. southwest of the continent. On these deserted and sandy lands one can see, not only most of the endemic genera, but also the great genera of Australia. Some cosmopolitan tropical genera are also represented, but so modified that they show very little resemblance to their tropical relatives. There are examples of these in the phillodinic acacias: Phyllanthus and Cassia.
The two most numerous genera are Acacia with 412 species and Eucalyptus with 230 species. Of the 500 tropical and subtropical species of Acacia, 300 form the Phyllodineae and are characteristic of Australia and Polynesia. In these the pinnate leaves are replaced by simple leaf-shaped laminae, derived from the flattening of the petioles and the main veins. These phyllodes are flattened so as to arrange their surfaces parallel to the incident rays of light and thus reduce transpiration.
Most acacias are xerophilous. In the arid or semi-arid parts certain species of acacia grow together in bushy groups and give a type of vegetation which is known under the name of scrub. The mulga scrub (Acacia aneura) is characteristic of large areas of land, in which the annual precipitation does not exceed 380 mm.: it forms part of the peripheral savannah of semi-arid Australia and is the main component of the bushy steppe (shrub – steppes) of arid Australia. Other examples of scrubs are the brigalow (Acacia harpophylla) and the gidgee (Acacia Cabbagei). In the scrub area, shepherding is practiced above all; indeed the mulga is of great nutritional value as a fodder.
The Eucalyptus genus is almost entirely limited to the Australian continent, where in fact 230 species have been collected while no more than two or three come from Insulindia. It is one of the most characteristic Australian plants, easily recognizable by the operculum of the floral button. The Eucalyptus are mostly trees or tall shrubs, and some species reach enormous sizes. It is officially established that Eucalyptus regnans on M. Bawbaw (Melbourne) reaches 99.4 m. tall and almost 9 m. in circumference to 2 m. above the ground: this height is higher than that mentioned for Sequoia gigantea, although it is still lower than that of Sequoia sempervirens, which reaches a height of 104 m. (Willis, VII, p. 604).
In the juvenile state the leaves of Eucalyptus are opposite and dorso-ventral, but later they become alternate and isolateral, a modification more suitable for xerophilic conditions. The bark varies a lot and this variation is of valid help in the classification: smooth bark (rubber trees) that peels in pieces, scaly bark belonging to the bloodwoods, compact bark with longitudinal fibers (stringybarks), hard and furrowed bark that turns black with age (ironbarks). Due to their rapid growth and their often valuable wood, Eucalyptus are now cultivated in various parts of the world. The Eucalyptus they constitute a xerophilic genus, which thrives well and has spread throughout the Australian continent: it avoids the brush – forest of the eastern region but is found in the xerophilous flora close to the sandy areas; it then forms on its own large forests, a particularity of which is the great development of a xerophilous undergrowth. The Eucalyptus also dominate in the peripheral savannah; but in deserted and arid Australia they are restricted to the regions where the humidity is higher: such as the sandy beds of the watercourses and the flat clayey soils subject to flooding after the rains.
An interesting form of development of Eucalyptus is the mallee (oily Eucalyptus, E. Dumosa, etc.) That forms dense scrubs in various parts of Victoria, South Australia and New South Wales. In it are formed several trunks instead of a main one and the plant supports a compact umbrella, a characteristic aspect of the shrubs of arid regions. The aquifer tissue is especially developed in the swollen roots which were used by the natives for drinking in times of drought. Now the mallee scrub has disappeared from many southern regions and replaced by land reclamation and grain crops.
Among the forest species of Eucalyptus there are some that give very hard and precious woods; first of these are the Yarrah (E. marginata) and Karri (E. diversicolor) characteristic of Western Australia’s forests.