4 # 
The Colorado Experiment Station 
the air, moisture of the air, precipitation (rainfall and snowfall), 
wind, and atmospheric pressure, are those which work over areas 
of great extent. Local factors, such as temperature of the soil, 
water and air in the soil, texture and composition of the soil, life 
of the soil, direction of slope, degree of slope, etc., are those which 
work within rather narrow limits and are determined by local 
physical conditions. Differences in the character of the vegeta¬ 
tion of a certain climatic area are determined by the local factors 
just mentioned. For example, the climatic type of vegetation 
over the Great Plains is that of a dry grass-land (grass-steppe). 
Within this large climatic grass-land area, there are local varia¬ 
tions in the plant covering which may be due to differences in soil 
texture, slope, exposure, *etc. For example, in soil readily pene¬ 
trated by water, Psoralea, a deep-rooted plant, is locally very 
abundant; again, in low, seeped places, alkali plants may prevail, 
and on shale knolls, Yucca be locally dominant. 
There are many possible combinations of factors that may 
operate upon a plant. In a state like Colorado, with its varied 
climatic conditions, its slopes and exposures of all degrees, its 
highly varied geologic structures calling forth many soil types, 
these combinations become innumerable. Each combination of en¬ 
vironmental factors calls forth corresponding activities in the 
plant life; and these activities are represented, in a practical way, 
by the quality and quantity of the crop yield. 
MaPs Control of the Environmental Factors. —Man, in his 
agricultural pursuits, has attempted, and often successfully, to 
artificially modify the external conditions about the plants he is 
growing. He may decrease the light intensity by shading or thick 
planting, increase the light intensity by thinning the plants, 
lengthen the duration of light by the use of artificial light, as is 
sometimes done in the forcing of vegetables in the greenhouse, 
and may even change the quality of the light by the use of stained 
glass, although such a practice is not practical. He may increase 
the temperature of the air about plants by the use of hot-houses, 
hot-beds, and by different methods of orchard heating, and de¬ 
crease the temperature of the air by cold-houses, and by shading. 
Moisture in the air is increased by housing, Fy shading, by plant¬ 
ing so as to have a thick stand, and by having windbreaks; and 
the moisture may be decreased by housing and by having a thin 
stand. Man may modify the effects of wind movements by.wind¬ 
breaks and by increasing the density of crop stand. Local factors 
are also more or less under control by man. The structure of the 
soil may be modified by cultivation and by the application of 
