54 PLANT HABITS AND HABITATS IN THE 



the upper one. In the vicinity of Oodnadatta, however, the soil 

 solution of the lower plain probably carries a larger percentage of salts, 

 which, as will be remarked below, makes for physiological dryness. 

 The sandhills probably constitute excellent reservoirs for water, and 

 through the rapid drying-out of the surface of the soil the capillary 

 chain is broken and evaporation into the air above the ground is either 

 wholly stopped or at least it is very decidedly retarded and reduced 

 to a quantity that for practical purposes can be neglected. On the 

 other hand, there are no indications of water erosion among the sand- 

 hills, and such probably does not occur. 



Unlike soil of a fine physical character, as, for example, that of the 

 plains, prompt penetration of the rains takes place even if the soil 

 at the time is air-dry and none is lost by run-off. The dust mulch 

 on the surface is probably 30 cm., more or less, in thickness, and the 

 soil beneath, of homogenous sand, is moist to a depth dependent only 

 on the amount of the rains. The only water lost from such depths is 

 through the plant covering. The sandhills, therefore, have water 

 relations which are considerably more favorable than would appear 

 from a superficial examination. 



In the other areas, such as have poor surface drainage, the amount 

 of water received and also in certain instances retained must be said 

 to equal, if not to exceed, that of any of the other areas. In such 

 areas the degree of aridity is related to the concentration of the soil 

 solution. In the case of the clay-pans, for example, when first flooded, 

 as previously mentioned, the water is fresh, or but slightly brackish, 

 and this is indicated by the character of the plant and animal life 

 to be found in them at the time. As the water escapes by evapora- 

 tion, however, the salts become more and more concentrated until 

 only such forms as are especially adjusted to withstand such dense 

 solutions are able to survive. Carrying possibly not a small amount 

 of moisture, such areas are nevertheless intensely arid, speaking in a 

 physiological sense. 



From this sketch of the leading physiographic features of the 

 vicinity of Oodnadatta, it can be seen that there are about four or five 

 well-defined plant habitats. These are the upper and lower plains, 

 the flood-plain of Neales River and its branches, the sandhills, and 

 the clay-pans, with which might be included other areas having poor 

 drainage. As will be shown later, these areas have for the most part 

 characteristic vegetation, as might be expected from their marked 

 differences, some of which have been outlined above. 



