Instant Notes: Plant Biology

evidence that about 120 million years ago there were only two continents and

the first flowering plants were appearing. The subsequent evolution of

flowering plants has been dramatically affected by the shifts in continents that

have occurred since, and consequent isolation or joining together of, large land

masses. The regions are mainly similar to vertebrate geographical regions but

are less well defined owing to the fact that flowering plants evolved before

modern vertebrate groups and migrated across the world earlier, maintaining

affinities across regions. They differ particularly in the southern temperate zone.

The whole north temperate zoneis regarded as a single region, the boreal.

The north Atlantic formed about 50 million years ago and North America and

Eurasia have periodically been joined at the Bering Strait. The whole region has

had great climatic changes in the last million years through glaciations, prob-

ably extinguishing many species so that only adaptable plants with good

dispersal powers remain. In the tropics there are three regions: South America

with central America; tropical and subtropical Africawith tropicalAsiaforming

the palaeotropical region; and Australia. The Caperegion of South Africa is a

separate and exceptionally diverse small region and the Antarctic region

comprises southern South America and New Zealand.

There are strong affinities between all the tropical floras; New Guinea and

north-eastern Australia have a similar flora to Asia. The southern super-conti-

nent,Gondwana, split up about 100 million years ago to form the southern

continents of today and all drifted north except for Antarctica. The Antarctic

region retains a remnant flora of what was once widespread across Gondwana,

and the Cape and Australian regions have rather different Gondwanan floras of

drier Mediterranean-climate and semi-desert floras that have evolved

separately. They have not been subject to the major glaciations and climatic

shifts of the northern hemisphere. Minor phytogeographic regions are formed

by some oceanic islands, particularly in the Pacific.

Over the last million years, periodic cooling and warming of the world has led

to glaciers spreading and retreating in the northern hemisphere and the tropics

becoming drier during glacial periods and wetter during interglacials. These

Glaciation and
plant migration

K1 – Physical factors and plant distribution 165

0 °

Neotropical

Antarctic

Cape

Palaeotropical

Australian

Boreal

Fig. 2. Phytogeographic regions of the world.