CHAP, vn.] DISTRIBUTION OF PLANTS. 195 



abundant in the region indicated, but not at the same 

 time monopolizing the entire district. 



The latitude of a place does not necessarily tell the 

 range of temperature, the isothermal lines, or those 

 running through places having the same mean annual 

 temperature, are in the region of the equator more or 

 less parallel with the lines of latitude, but as we recede 

 from the equator the two are widely separated, the 

 divergence depending on the proximity of the ocean, 

 mountain ranges, etc. The annual isotherm of 50'^ 

 passes through'- latitude 42° 30' in eastern America, 

 51° 30' in England, and 40° in Eastern Asia ; nevertheless 

 the floras of these places are widely different, depending 

 on the fact that the mean annual temperature may be 

 the same in a place having a moderate summer and 

 winter temperature as in another having a very hot and 

 short summer and a very long cold winter. The last 

 mentioned distribution of heat is productive of a short 

 and sudden burst of floral glow as seen in the Swiss 

 Alps, or in high latitudes, as in Iceland, whereas the 

 first mentioned distribution of temperature favours a 

 prolonged activity of the different members of a flora, as 

 seen in England, 



Light has already been shown to be indispensable to 

 the existence of all plants whose assimilation depends 

 on the presence of chlorophyll, but at the same time 

 this factor to some extent influences the distribution of 

 plants. Some kinds require full exposure to light j 

 others flourish best in the shade. 



The varying density of the atmosphere at different 

 elevations appears to produce little or no effect on plant 

 life, nor to be the cause of any morphological or physio- 



