554 



occurs in fchose parts where the soluble salts in the soil 

 reach a high concentration. None of the species of the 

 "saltbush" or "bluebush" vegetation of the plains, which 

 have so often been termed haloph3rtes bv other workers, were 

 present around the lake, with the exception of .4 triplex vem- 

 cariutn in small quantities near the extreme margin. The 

 Atriplex of the salt lake was Atri'plea- ji^^^if^osiim, which also 

 occurs in coastal salt swamps ; it is a true halophyte. The fact 

 that halophytic vegetation is composed of members of the 

 Chenopodiaceae, and often of species of the same genera that 

 occur on the Nullarbor Plain, cannot be taken as proving that 

 the character plants of the latter habitat are halophytes, as 

 was assumed by Cannon, This distinction between the halo- 

 phytes of a salt lake and the ''saltbush" or "bluebush" com- 

 munities has previously been noted by Tate (loc. cit.J. We 

 regard it as an important one. 



In any climate where the evaporation rate is in excess of 

 the precipitation the soil will tend to accumulate considerable 

 quantities of soluble salts. But in no part of the Nullarbor 

 Plain was there any sign of surface accumulation of crystals. 



The communities of Kochia and Atriplex that occupy the 

 surface of the Nullarbor Plain with their rich crop of annuals 

 ought to be regarded as representing a semi-desert flora rather 

 than a halophytic one. The bluebush, and to a less extent 

 the saltbush, seem to represent the succulent flora for this 

 region. Comparisons would be better made between them 

 and the constituents of the succulent communities of South 

 Africa or America than with halophytic plants. 



On the Nullarbor Plain the vegetation of the dongas, with 

 the scattered small trees and bushes, represents a further 

 advance that takes place in that area with increasing amounts 

 of soil, and especially of moisture. The communities of the 

 dongas were surprisingly like those developed on the sandhill 

 hollows. They appear to exist under very similar conditions, 

 though the vegetation is much more sparse and less advanced 

 owing to the more severe environmental conditions. Con- 

 sidered in relation to the plain formation as a whole these com- 

 munities of plants in the dongas appear to represent what 

 Clements (i^) would term a post-climax : that is to say, while 

 the general climatic and edaphic conditions cause a stoppage 

 of development on the plain at the stage of an open com- 

 munity of bluebush, in the slightly more favourable conditions 

 in the dongas the process is carried to a further stage. This 

 further development can be traced to some extent; in the 

 smallest depressions only very xerophyllous species occur, e.g.^ 

 Acacia tttragonopliylla ; larger depressions with more soil have 



(]5) Plant Succession, 1916, p. 109. 



