12 



WILD FLOWERS AND TREES OF COLORADO 



tude" in a very loose way. It is worth while finding out what this 

 really means. To most people thinness of the air is the first thing thought 

 about in connection with high mountain districts. It makes breathing 

 difficult and a person is soon "winded" by slight exertion. But this 



Fig. 12. — Vegetation of the Mesas 

 The view shows the western part of two mesas next the foothills. Pine trees form 

 an open forest on the top of the mesa while the sides are bare. In the gulch there are 

 thickets of willow and thornapple. 



thin air does not disturb plants. Indeed there have been experiments ' 

 made upon plants in laboratories which show that they grow very well 

 in glass cases when a part of the air is removed and make a larger and 

 faster growth than when under ordinary conditions. Hence, it is apparent 

 that the dwarfing of high-altitude plants is not due to rarity of atmosphere. 



