I lO 
THE AMERICAN BOTANIST. 
tion, and it is on their distribution that the distribution of vegeta- 
tion is principally dependent. The influence of heat and moisture 
is noticed as we pass from the equator to the poles, or from the 
base of a tropical mountain to its summit, thus arises a horizontal 
and a vertical distribution of vegetation." 
The horizontal zones of vegetation have heretofore been desig- 
nated the tropical, subtropifcal, warm temperate, cold temperate, 
subarctic, and arctic, each with its distinguishing climate and veg- 
etation, although overlapping each other somewhat. The Biolo- 
gical Survey, however, has given a different division, calling the 
three life zones of North America the tropical, austral, and boreal, 
subdividing these into the tropical, lower austral, upper austral, 
transition, Canadian, Hudsonian, and arctic— seven in all. The 
tropical is indicated by the region in the extreme southland, includ- 
ing the southern parts of Florida, Central America, the lowlands 
of Mexico, and the lower Gulf region ; the austral covers the whole 
of the United States and Mexico, except along the extreme north 
and south, while the boreal covers the entire northern part of the 
continent to the polar regions, and also a few peaks of the higher 
mountain systems within the United States. 
Subdivided and considered for temperature, the survey finds 
that the tropical zone is confined within that area which has a total 
quantity of heat approximating at least 26,000 degrees F. This 
is obtained by considering 43 degrees as the beginning of plant ac- 
tivity. With 43 degrees as a beginning, the effective temperature 
or degrees of heat (daily mean normal shade temperatures) in ex- 
cess of this number were added together for the several stations 
considered in the different zones, beginning when the normal 
means in spring exceeded 43 degrees and continuing until it fell 
to or below that point in the fall. With this as a basis the tropical 
was found to lie within lines which were bounded by an annual 
quantity of at least 26,000 degrees. The lower austral was con- 
fined to a region which required a total quantity of at least 18,000 
degrees F. The upper austral to a region and to plant species 
which required at least 1 1,500 degrees F. The plants of this zone 
apparently can not endure daily a summer temperature during six 
consecutive weeks of 79.8 degrees. The transition was confined 
