6 TREES AND SHRUBS 



Alkali is also very apt to be present in greater or less quantity 

 is such soils. 



These generalizations as to the soils of the State only 

 show their variable nature and make necessary a careful con- 

 sideration of the particular soil, in which we wish to grow 

 plants. 



Our State is all of it relatively high above the sea, its 

 lowest valleys being more than 3500 feet above that level, 

 while some of the mountains rise considerably above timber 

 line. This elevation, when taken in connection with our geo- 

 graphic position, produces climatic conditions in atmospheric 

 humidity, temperature variations and extremes, and a light 

 intensity that are especially severe on all plant life except that 

 which is particularly adapted to such conditions. The lack of 

 moisture in the air is particularly hard upon plants that are 

 accustomed to a humid atmosphere, since it increases the 

 evaporation from their leaf surfaces to such a degree that they 

 are able with great difficulty to get enough water from the 

 ground to replace that lost by evaporation. 



Temperature changes, both diurnal and annual, are ex- 

 treme with us, because our altitude renders our atmosphere 

 thin and the extreme dryness but accentuates this effect. Ob- 

 jects in direct sunlight are quickly heated to a high temper- 

 ature, while as soon as the sun is down, all temperatures drop 

 suddenly and severely. Our growing season is very long 

 and the total quantity of heat received during a season is very- 

 great, a fact tending to produce rapid growth and relatively 

 short life in our trees. 



Light is a necessity to all self-supporting plants (i. e. not 

 parasitic or saprophytic) : they must have light in order to 

 manufacture their food. Very few people seem to appreciate 

 this necessary condition of plant life. But plants must have 

 light in the proper quantity, and this quantity differs fully as 

 much as does the proper quantity of water which they re- 

 quire. Some species will grow only in strong light (i. e. are 



