ee 
Bes D He 2 ors sie 
PE Sy erie 
1903] FLORA OF NORTH CAROLINA 251 
logic constitution of these plants is taken into consideration; for, 
according to the laws of temperature control of plants, species 
of neotropic and warm temperate origin need a large sum of heat 
units to carry on adequately their life-processes, and these are 
not properly carried on unless the specific number of heat units 
is provided in the environment. 
INFLUENCE OF GLACIERS ON THE FLORA OF NORTH CAROLINA. 
Upon the retreat of the ice-sheet, that portion of the con- 
tinent north of the terminal moraine was tenanted again by 
plants that migrated northward, which as species were adapted 
toacold temperate climate. A large number of these speciés came 
from the southern Appalachians and adjoining regions, where 
they had remained undisturbed in their original haunts during 
the long ice age, and were in a plastic condition for growth ina 
new environment, through the influence of the pressure of species 
upon each other in their southern home, and through the physio- 
graphic vicissitudes to which these fictile forms were subjected. 
Many species, therefore, growing in the North Carolina moun- 
tains, found congenial conditions in the more extensive land 
areas in the north. 
An inspection of the forest maps to be found in the ninth 
volume of the Tenth Census Report, Forest Trees of North America, 
will show that there is a center of distribution which compre- 
hends the area of the present states of southern and central 
Pennsylvania, West Virginia, Kentucky, Tennessee, western North 
Carolina, southwestern Virginia, northern Georgia, Alabama, 
Mississippi, where the largest number of species of the most 
important genera of North American deciduous trees will be 
found. A study of these maps reveals an important fact, that 
the spread of the species from this common center has been 
in a series of more or less concentric waves. Three waves may 
be distinguished. The first wave consisted of the distinctly 
glacial flora, which skirted the border of the ice-sheet. The 
second consisted of the present boreal forms, and the third was 
a wave of deciduous shrubs and trees, oaks, hickories, and a 
: host of others. The species most successfully provided with 
means of distribution and most easily adjustable extended furthest 
