742 TEXTBOOK OF BOTANY 



spring, where light intensity is never high, and the daily light period is 

 24 hours long during the short growing season. The southern extremity 

 of the continent lies within the tropics, where, on the basis of tempera- 

 ture, there is but one season, summer, which may be wet or dry or pe- 

 riodically wet and dry, where light intensity is high, and the daily light 

 period is 12 hours long throughout the year. 



A west-to-east belt across the continent from Oregon to Massachusetts 

 is the region of prevailing westerly winds and is marked by great differ- 

 ences in total precipitation, in snowfall and rainfall, in the prevalence of 

 drizzling rains or sudden downpours, and in seasonal distribution of pre- 

 cipitation. These variations affect the atmospheric humidity and the 

 available soil water. Starting at the west coast, with a hundred inches of 

 rain principally during the colder months of the year, and persistent 

 fogs, the high coastal mountains lead to a reduction in rainfall on their 

 eastern slopes, so that grassland and semi-desert conditions prevail on 

 the high plateau. Precipitation again increases in the Rockies, but the}' 

 in turn cast a rain shadow eastward on the high plains, and 10 to 15 

 inches of precipitation is general. The annual precipitation in the eastern 

 half of the United States varies from 30 to 60 inches and comes from the 

 moisture in the air masses moving in from the Gulf of Mexico. Central 

 America extends into the trade winds, or prevailing easterlies, and rains 

 are abundant east of the mountain crests and meager on the western 

 slopes. 



As a result of the many possible combinations of light intensity, length 

 of day, temperature extremes, length of growing season, rainfall, snow- 

 fall, frequency and length of drought periods, fogs, and hot dry winds, 

 the continent as a whole has many climatic regions, large and small, ex- 

 tending in various directions; and the nature of the vegetation reflects 

 the diversity of conditions. Indeed, students of climates have long recog- 

 nized that the larger patterns of vegetation are among the best indicators 

 of climatic patterns and boundaries. High mountain slopes and crests 

 may approximate conditions of high latitudes, while protected rivers 

 with warm seacoasts in the north may simulate conditions of the south. 



Prehistoric factors. In addition to the amount and intensity of present- 

 day factors, the distribution of many plants has been limited by prehis- 

 toric or geologic factors. Among these factors, slightly lower temperature 

 and glaciation, with their attendant effects on precipitation, drainage, 

 and soils, have left their imprint on the vegetation of many parts of the 

 continent. Climatic shifts involving prolonged periods of drought or of 



