The Vegetation of Western Australia. 
li. 
old pan*Australian element is maintaining its last stand, or on the other hand, 
and more probably, that it is in this environment that as an unmodified in- 
vading element the autochthonous flora finds conditions suitable to its de- 
velopment. I have previously pointed out that most of the plants of the 
Autochthonous element are psammophytes. 
On the other hand, the loamy and alluvial soils of the Eremea provide 
the environment for the hardier species of the Palaeotropic Element. The 
species of Sorghum, together with many other palaeotropic grasses, Acacia 
Farnesiana, Clerodendron, the Cucurbitaceae, etc., have established them- 
selves in regions far south of the Tropic of Capricorn. For example, Acacia 
Farnesiana occurs as far south as the Murchison Kiver, and Ficus occurs as 
a lithophyte as far south as Mount Margaret. 
THE FORMATIONS. 
(a) THE SCLEROPHYLLOUS WOODLAND. 
The sclerophyllous woodland of the Eremea is the same in composition as 
that of tlie marginal areas of the South-West Province, differing only in the 
increased paucity of its undergrowth, and in the fioristic composition, as well 
as in the importance of grass, which increases as warmer temperatures are ex- 
perienced, while the increasing aridity results in greater numbers of Cheno- 
podiaceae in the open spaces. A brief description of the formation has already 
been given. It is sufficient to say here that the formation extends furthest 
in saline and loamy soils, the most inland locality being found near the sliores 
of Lake Darlot, where a Eucalyptus woodland of E. Duyidasi and E. Le-Soueffii 
occurs in saline soil with a ground covering of Chenopodiaceae, Myoporaceae, 
and Malvaceae, including the remarkable Plagianthus Helmsii. 
(b) SALSOLACBOUS SHRUB STEPPE. 
To the east of Israelite Bay and Balladonia, there is an extensive area 
known as the Nullarbor Plain, the limestone soil of which supports a vegetation 
averaging 50-100 cm. in height, consisting chiefly of Chenopodiaceae, Myopor- 
um, Eremophila, and other low shrubs which are distinctly Eremean in cliarac- 
ter. During wet periods a remarkable herbaceous growth cpiickly appears, 
to die again when the soil moisture is exhausted. I have no first hand know- 
ledge of the formation, but apparently it is developed extensively in South 
Australia. Its ephemeral herbaceous species are mainly Compositae and a few 
annual grasses. 
(c) MULGA BUSH. 
The mulga bush covers a greater part of Australia than any other single 
formation. The conditions limiting its distribution to the south are only 
imperfectly understood, but the line corresponds roughly to the Meyer Ratio 
isopleth of 25 inches, and more approximately to the 80° F. mean isotherm 
for January and February (the end of the dry season). There is also a certain 
change in the soil which may be the result of climatic factors, and a certain 
pluvio-thermal significance in this line, since to the south, with a slightly 
increased rainfall, the wet season extends from March to August, while in the 
mulga bush, the six consecutive wettest months are January to June. The 
annual rainfall falls short of 950 points. Little more can be said of the climatic 
control except that the mean temperature for the four wettest months is in 
excess of 18° C., while for the sclerophylous woodland to the south it is less 
than 18°. Unfortunately there are few meteorological stations within the 
area, and only two near the boundary, and thus our knowledge concerning 
the controlling climatic factors is very limited. The line of demarcation is, 
