ARID PORTIONS OF SOUTH AUSTRALIA. 



113 



A "composite" leaf constructed upon the average length and width 

 of the leaves of 30 representative species growing in the desert-semi- 

 arid regions of South Australia has a length of 64 nun. and a breadth 

 of 5 mm., giving the length-breadth ratio nearly 13 to 1. Mea'sure- 

 ments on the phyllodia of 16 species of Acacia give an average length 

 of 84 mm. and an average width of 3.5 mm., or a ratio of length to 

 width of 24 to 1. The average leaf-area was not determined in either 

 class. In another series of 29 species both leaves and phyllodia were 

 measured and the area computed. In this case the ratio of area to 

 length was 4.7 to 1. For the leaf sizes observed, it would appear, 

 therefore, that the ratio of area to length is greatly under that to be 

 expected in species growing under moist conditions — that is, in species 

 which bear leaves with equal transpiring surface, or, in other words, 

 with better developed lamina. The length-width and the area-length 

 ratios, for this reason, may possibly constitute an index of the degree 

 of xerophylly of the species. Measurements of leaf and phyllodium 

 sizes are given in table 16. 



