34 



is the common way in which a plant enlarges the surface ; 

 but there are other factors to be considered. Plain surface 

 means rapid evaporation, and this loss of water has to be 

 made up by correspondingly rapid absorption by the roots. 

 If the evaporation from the leaves is in excess of what the 

 roots are able to absorb and pass up through the stem, wilt- 

 ing and cessation of function will take place. Not only 

 that, but it is found that too rapid a passage of water is 

 itself injurious. Also the green tissue of a leaf is a very 

 delicate organ that will not bear too free an exposure to 

 intense sunlight. Therefore a plant with a copious delicate 

 foliage is ill-suited for growth in a very sunny, dry district. 

 Conversely, we should expect to find in such a district that 

 the plants consist of forms with reduced means of evapora- 

 tion and leaves modified to protect their delicate tissue. 

 Austialia as a whole is far too sunny and of too dry an> 

 atmosphere to support such a vegetation as would thrive 

 in Europe. We therefore have a preponderance of plants 

 well suited to our conditions. Leafless plants are common. 

 Plants whose leaves are much reduced in surface and of 

 thick consistency are much more common, while our Euca- 

 lypts compromise by hanging their leaves up and down 

 to escape the full force of the sun's rays. The Acacias, 

 whose remote ancestors without doubt had extensive leaf 

 surface, have adapted themselves to Australian conditions. 

 Some have no leaves : in others these organs are much 

 reduced ; and in all structural changes have taken place 

 to reduce evaporation and the penetration of too intense 

 light. 



The three great families typical of the Australian bush, 

 the Myrtles, Proteas, and Legumes, are conspicuous in the 

 structure of their leave?. If these organs are not reduced 

 greatly in surface their thick, leathery consistency has 

 the same effect. This gives to our landscape the peculiar 

 sombre appearance so much in contrast to our sunny skies. 



