Earth Sciences / 85Nevertheless, regions of the world can usually be defined in terms of the plants occurring naturally
within them and those regions coincide, more or less, with the climatic zones. The plants growing in
a particular area comprise the ‘flora’ of that area and floras can be grouped into units, called
‘phytochoria’ (singular phytochorion), in which small unrelated floras, such as the Lusitanian in
northern Europe, are designated ‘elements’. Once defined, phytochoria can then be grouped further
into a hierarchical system. The highest category is the floral realm or kingdom (both terms are used),
which is divided into regions. Regions are subdivided further into provinces or domains (the terms
are synonymous), each comprising a number of districts. Some classifications allow intermediate
ranks and subdivisions of districts (MOORE, 1982, pp. 210–219). Realms are identified by the
presence of particular plant families, regions by the presence of 20–30 per cent of plant genera that
are not found elsewhere (i.e. endemic genera), and provinces by their endemic genera.
Most classifications recognize four floral realms: Holarctic, Palaeotropic, Neotropical, and
Austral. The Holarctic Realm comprises North America, Greenland, Europe, and Asia except
for India and the south-west and south-east (which became attached to the main landmass
during the Tertiary). Floristically, the mountains extending from the Atlas range in North
Africa across southern Asia to the Himalayas mark the southern boundary of the northern
hemisphere in the Old World. With a few exceptions, coniferous trees occur north of the
boundary and palms to its south.
The Palaeotropic Realm (the name means ‘old’ tropical) comprises Africa south of the Atlas Mountains
except for southern Africa, Madagascar, Arabia, southern Asia including India, and the islands of the
tropical Pacific. The Neotropical Realm (‘new’ tropical) comprises Central America including the
southern tips of California and Florida, the Caribbean, and most of South America. Although their
climates are similar, floristically these tropical realms differ from one another markedly because of
the time that has elapsed since continental drift separated them. Cacti, for example, are characteristic
of the New World and are one of the defining families (Cactaceae) of the Neotropical Realm; those
found in the Old World have been introduced. This is why they are regarded as two distinct realms,
rather than one.
The Austral (or Southern) Realm comprises the southern part of South America, southern Africa,
Australia, New Zealand, and the islands of the southern Atlantic and Pacific. Here, too, the landmasses
are now isolated from one another. Southern Africa and southern South America differ floristically
from the rest of the continents to their north and share some plant families with Australia and New
Zealand. On this basis they are grouped together as one floral realm or, in some classifications,
ranked as individual Australian, Cape, and Antarctic Realms. Figure 2.30 shows these realms and
the 30 floral regions of which they are composed.
Floristic realms and regions vary in size, but all are vast and difficult to comprehend. It is not until
their subdivision reaches the provincial level that they become easily recognizable. Western Europe,
for example, from northern Spain to Denmark and the Norwegian coast, constitutes the Atlantic
Province. The Boreal Province, supporting vast tracts of coniferous forest (known in Russia as the
taiga) forms a belt across Europe and Asia between the Ural River and Gulf of Finland and latitude
60° N. The North American equivalent, covering most of Alaska and Canada south of the Arctic, is
called the Hudsonian Province.
Animal distribution is also described geographically and, because particular animals are often
associated with particular plants, zoographical and floristic realms almost coincide. The concept of
realms, with their subdivisions, should not be confused with that of biomes, which are defined
ecologically. Floristic classification reflects climates, past and present, and the history as well as
present geography of the planet.