16 



PLANT HABITS AND HABITATS IN THE 



The average relative humidity (3 p. m. readings) for the year 1912 

 for arid-subhumid stations, compiled from Hunt (1912), illustrates the 

 wide distribution of the low relative humidities. The average for 13 

 arid stations gives a relative humidity of 32.3 per cent. The average for 

 a like number of semi-arid stations is 37 per cent. And, finally, the 

 average for 9 subhumid stations is 42.1 per cent. 



Fig. 7a. — Mean humidity of Australia for January, after Taylor, 1918. 



In summer, when the center of minimum relative humidity is 

 farthest south, practically all of South Australia is included within the 

 50 per cent relative humidity isopleth, and possibly in over one-half 

 of the state at this season the relative humidity averages 40 per cent 

 or less. 



Light. 



The amount and quality of light to which plants are exposed, as is 

 well known, are extremely variable. The light in humid regions, for 

 example, is less intense and contains less of the more refrangible rays, 

 the blue, violet, and ultra-violet, than that in arid regions. The light 

 in high latitudes is weaker than that of the tropics. From its geo- 

 graphical position, as well as from the fact that much of the continent 

 ia relatively and actually arid, it follows that much of Australia is 

 exposed to an intense light, rich in actinic or chemical rays. In the 



