ARID PORTIONS OF SOUTH AUSTRALIA. 123 



Acacia tetragonophylla. 



In Acacia tetragonophylla the phyllodia are needle-shaped and short. 

 Moreover, in especially dry habitats or during dry seasons they may 

 be largely shed, thus reducing the transpiring surface. An analogous 

 condition was observed in A. colletioides, having similar phyllodia, 

 at Ooldea. On the other hand, where the environment is relatively 

 moist, as in certain habitats at Copley, the phyllodia appear to remain 

 for several years. 



The leading features of the structure of the phyllodia of the species 

 (fig. 16) include prominent masses of sclerenchyma, a part of which 

 project outward from the conductive tissue to the epidermis, and a 

 part between it and the central parenchyma, the central non-chloro- 

 phyllous parenchyma, and the masses of chlorenchyma which lie 

 exterior to the latter and between the peripherally placed scleren- 

 chyma. The cuticle is fairly heavy. No trichomes or resinous secre- 

 tion were observed. Thus there appears to be a striking absence of 

 certain features clearly related to the extremely arid environment in 

 which the species occurs. However, the fact that the phyllodia are 

 of small size and fall away readily may be a reasonable explanation 

 of this condition. The relatively large proportion of cells with heavy 

 walls is, as has already been referred to, a direct expression of imme- 

 diate effects of an arid environment and is not related directly to the 

 storage or to the husbanding of the water-supply. 



