The Vegetation of Western Australia. 
Ivii. 
eroded sand, and gibber-plains on wliich there is no vegetation of a permanent 
kind. The only trees met with are in the dej^ressions or at the foot of the 
escarpments, the most common being Hakea lorea, the desert shcoak {Casuar^ 
ina Decaisneana), Eucalyptus gamophylla, and E. setosa, the two last-named 
rarely exceeding 10 or 12 feet in height. A few plants of mulga occur liere 
and there in the depressions, e.g.. Acacia sibirica and A. Kempeana, while in 
the heavier soils are a number of Chenopodiaceae, such as Kochia, Bassia^ 
and occasionally Atriplex. Following adequate precipitations, an ephemeral 
flora is born, much resembling that of the savannah and Mulga-bush country, 
but it disappears after completing its brief life cycle. 
The sand ridges are usually scantily clothed with shrubs, amongst which 
Crotalaria Cunninghamiiy NeiOcastlia, and Duhoisia Hopwoodii are the most 
common, while on the flat sandy intervening country are occasional shrubs 
of Hakea rhomhalis, Orevillea eriostachya and G. juncifolia, Calothamnus, Dicra- 
styles, Thryptomene Maisonneuvii, Dampiera, Petalostyles, Hibiscus pinonianvs, 
Brachysema Chamhersii, Sida and Acacia, Cassia desolata, and Micromyrtus. 
Tt is interesting to find here such plants as Dampiera, Brachysema, Colo- 
thamnus, Thryptomene, and Micro^nyrtus. They are all local endemics derived 
from a south-western stock, and are psammophytes equipped for a hostile 
environment. Crotalaria, Hibiscus, and Petolostylis are of northern origin, 
and their appearance may seem strange, were it not for the fact that all possess 
an indumentum which enables them to thrive far removed from their main 
centres of distribution. One should perhaps in this connection mention 
Gossypium Sturtii, although we do not know whether this species is a psammo- 
})hyte, or confined to rocky gullies. 
The only endemic genus of the true desert is Newcastlia, which through 
Hemiphora, Lachnostachys, and Dicrastyles exhibits a certain aftinity with 
the South-West Province, and once again illustrates the fact that it is the 
psammophilous elements of the south-west which have not only exceeded the 
boundaries of the South-West Province, but have been able to create new 
forms in an arid environment. 
As the gradual process of continental desiccation progresses, so also is 
the desert extending. Under the influence of the prevailing winds it is grad- 
ually encroaching upon its neighbouring formations, thrusting here and there 
into the Triodia Steppe, and while it is extending to the coast in the vicinity 
of the Eighty-mile Beach, another arm of the steppe is extending along the 
Tropic of Capricorn to the coast, in the vicinity of the Ashburton Kiver. 
There can be little doubt that the Hamersley district is threatened with an 
Eromean invasion which will, sooner or later, completely overrun it. 
Vlll.— EPHARMOSIS AND GROWTH FORMS. 
Just as the species is the unit of the systematic botanist, so is the vege- 
tation form” the unit of the ecologist. To quote Warming : “ Every species 
must be in harmony, as regards both its external and internal construction, 
with the natural conditions under which it lives ; and when these undergo 
a cliange to which it cannot adapt itself, it will be expelled by other species 
or exterminated. Consec^uently, one of the most weighty matters of 
ecological plant-geography is to gain an understanding of tlie e])harmony 
of sj)ecies. This may be termed the plant’s growth form in contradistinction 
to its systematic form.” It reveals itself specially in the habit and in the 
form and duration of the nutritive organs, but shows to a less extent in the 
reproductive organs.” 
