zy MISSISSIPPI STATE GEOLOGICAL SURVEY [Bull. 
usually developed, but for which these plants would become 
rapidly exterminated. 
Mesophytes.—The usual, or what should be properly con- 
sidered, the normal conditions of plant life, are those of a 
moist air and soil which is appreciably moist, but not wet— 
water cannot be squeezed from it if taken into the hand. Such 
are the normal condition all about us in Mississippi. The great 
mass of vegetable life grows’ under these conditions, and the 
plants which constitute this mass are called Mesophytes, be- 
cause they live in relations to moisture intermediate between 
the two extremes of Hydrohpytes and Xerophytes. 
Examples of mesophytes are our familiar oaks, maples, 
beeches, and elms among trees, and the grasses and herba- 
ceous forms of the fields and woodlands. 
Heat.—In a large way Temperature is the most notable 
factor in distributing plant life over the earth. This is espe- 
cially observed in traveling from the tropics toward the poles. 
Temperature Life Zones run in broad belts around the earth 
roughly following parallels of latitude. Merriam has worked 
out the law controlling the limitation of these Life Zones. He 
has found that ‘‘plants and animals are limited in their north- 
ward distribution by the sum total of heat (above 6° C.) during 
the period of growth and reproduction,............ but they 
are limited in their southward distribution by the mean tem- 
perature of the hottest part of the year.’’* Conditions which 
modify this temperature will cause the limits of the zone to 
move northward or southward according to circumstances, so 
that in fact the zonal limits are far from being east and west 
lines. 
Merriam’ has recognized the following life zones in North 
America: 
ae ae: George Francis, A Textbook of College Botany, 1905, 
D. oO. 
*Mariam, C. Hart, Life Zones and Crop Zones, Bull. No. 10, 
S$. Bilogical Survey. Ais 
