ARID PORTIONS OF SOUTH AUSTRALIA. 85 



distinguishable habitats, and probably more — for example, the hol- 

 lows between the sandy ridges and the dongas on the plain. Study of 

 the plants of these habitats would undoubtedly show that each has 

 a very characteristic vegetation to be associated with the differences 

 which mark each habitat. 



Vegetation op the Nullarbob Plain. 



The eastern side of the Nullarbor Plain, as already mentioned, has 

 a very sUghtly undulating surface. When one is well out on the plain 

 he can see for many miles on all sides. No topographical feature 

 marks one direction as opposed to another. At first there is a sense 

 of barrenness, accentuated by the low relief, which is second only to 

 that to be felt in the region about Oodnadatta; a more careful exami- 

 nation of the plain does not fully justify this impression. A sparse 

 gray-green plant-covering, monotonous in form as well as in color, 

 is to be seen on every side. This is composed of halophytes of various 

 species, among which are chiefly Atriplex vesicarium and Kochia 

 sedifolia. There is also an ephemeral flora composed largely of grasses 

 which spring up after rains; but at the time of my visit (September), 

 only dried remains of such were to be found. These remarks on the 

 flora of the plain apply to the plain proper, or rather to the highest 

 ground, which composes by far the greatest percentage of its surface. 



Here and there, in looking about, one sees widely separated masses 

 of green of small extent, really oases, which appear quite Uke small 

 islands in a dull-gray sea. These are the dongas with their char- 

 acteristic vegetation. Such dongas as were visited between Watson 

 and Ooldea had a few low trees and shrubs and a fairly considerable 

 dead, herbaceous flora; but the plants, the woody forms especially, 

 appeared to consist of but a small number of species, although so few 

 dongas were seen that no dogmatic statement in this regard would be 

 warranted. Certain of these depressions near the Fowler's Bay road 

 •had Acacia tetragonophylla, A. aneura, and a "weeping sandal-wood," 

 possibly Pittosporum phillyrceoides. The kind as well as the abundance 

 of plants in the dongas illustrate in a very striking manner the very 

 great importance of the topography and of the substratum as factors 

 in the vegetation of an arid region. A similar condition is to be found 

 in southern Algeria (Cannon, 1913:31). There, for example, are 

 depressions (dayas) which are the centers of small drainage systems 

 in which there is an accumulation of soil and which have subterranean 

 drainage, so that an excess of salts in the soil solution appears not to 

 occur. Owing in part to drainage into the basins after storms, and in 

 part to the great amount of soil in them, constituting important 

 reservoirs, the dyas and dongas alike have relatively good water 

 relations and their fairly abundant flora follows as a matter of course. 



