116 PLANT HABITS AND HABITATS IN THE 



of a semi-fleshy type may be found in species of Xanthorrhoea, as is the 

 case in analogous forms elsewhere. The high water-retaining capacity 

 of the roots of certain woody species, as Gravillea stenobotrya and 

 other forms, is probably of great importance in the water-economy of 

 such plants. But the most striking of these must be considered to 

 be species of Eucalyptus which assume the mallee habit. In them the 

 short stem and the enlarged root-crown together constitute a very 

 remarkable water-storage organ which would seem to be capable of 

 holding sufficient water to carry the shoot over long periods of drought. 

 In such a bulbous organ it would be expected that the " feeding " 

 roots would be so situated that the utmost advantage would be taken 

 of the rainfall. This would mean the placing of the roots at such a 

 depth as would insure the absorption of water very soon after the com- 

 mencement of rains as well as the continuation of absorption of water 

 for a relatively long period after the rains were over. It is possible 

 that a further study of the roots of the mallees would show such root- 

 development, and in the species having the mallee habit there is a 

 tendency looking to specialization in relatively shallow roots, even if 

 under favorable conditions a considerable root-depth may be attained. 

 A study of species with a facultative mallee habit would be of interest 

 in this connection. 



The depth at which the superficial roots of perennials are placed is 

 variable, but in no case observed was it seen to be as shallow as those 

 of certain cacti in southern Arizona, for example. At Oodnadatta 

 the horizontal roots of Eucalyptus rostrata lie at a depth of about 

 60 cm. beneath the surface, but these may not be the most shallowly 

 placed roots of the species. Acacia linophylla of the sandhills east of 

 that place has superficial roots which run within a very few centimeters 

 of the surface, although the exact depth was not determined. At 

 Copley the most superficial laterals of specimens of Pholidia scoparia, 

 growing by a wash, were seen to lie at a depth of 40 cm., and the super- 

 ficial roots of Kochia sp., in a similar situation, were ascertained to lie 

 within about 10 cm. of the surface. At Quorn the superficial roots 

 of Eucalyptus leucoxylon var. pauperita were found to be 20 cm. 

 deep. 



At Copley some measurements were made on the greatest pene- 

 tration of the roots of annuals to be found at the time of my visit, 

 July- August, showing that for the most part they lie within 10 cm. of 

 the surface of the ground. The extreme depth observed was in the 

 case of a specimen of Zygophyllum crenatum growing on a slope below 

 high and rocky hills, where the soil was relatively coarse. Here in 

 one specimen a penetration of 13.5 cm. was found, although in two 

 others of the same species the extreme depth attained was 8 and 8.5 

 cm., respectively. 



