[Illustration: {uncaptioned}]
All of Colorado lying east of the base of the mountains, as well as
large areas in western Colorado lying along the course of the Colorado
River and its main tributaries, are within the life zone commonly known
as the Plains, and referred to in technical books as Upper Sonoran.
These areas are mainly below 5500 feet in elevation, and are relatively
flat. Clay soils are the rule, with local sandy spots. The rainfall
throughout this zone is scanty and irrigation essential to general
farming. These conditions have restricted the native vegetation
throughout this zone to species which can tolerate long periods of
drought, and thrive on sunshine with heat in summer and cold in winter.
A surprising number of species of flowering plants live and thrive on
these very conditions. We rarely find them in colorful masses, single
plants or small colonies being the rule.
Originally native grasses covered this zone with a fairly tight sod,
broken, however, by windblown patches and cut by arroyos. Live streams
were far apart. Trees were absent except for cottonwoods and a few box
elders along water courses. Settlement has brought roads, ditches,
cultivated fields and a large amount of livestock. These acts of man
have made life hard for some native flowers, but for most species,
living opportunity has been increased. The plains are flowerless only
for those who fail to pause and search.
The detailed growth patterns or specialized mechanisms by which the
various plains flowers resist drought, and so get a chance to live, are
numerous. In general they do one or more of these things: rush through a
short individual life cycle from seed to seed so timed that the new seed
crop is set before the heat of summer is far advanced; conserve the
limited moisture their roots gather by having few leaves and defending
them from animals by thorns or toughness; or, spend a large part of
every year, especially the dry, hot months, as a dormant bulb or buried
root stock.
The picture at the top of this page shows a plains area just at the base
of the foothills near Denver. It looks barren, but many species of
flowers can be found there in May and June.