CORK OAK-——-RYAN AND COOKE 
ceases, and the optimum temperature 
at which growth is most rapid. The 
optimum temperature lies somewhere 
between the minimum and _ the 
maximum. 
Plants, unlike animals, do not gener- 
ate their own heat and must, there- 
fore, obtain the heat necessary for 
their growth and existence from the 
air surrounding them and from the 
soil in which they are contained. It is 
a generally accepted fact that the 
temperature of plants and plant parts 
is approximately the same as their 
surroundings. Temperature, then, is 
the limiting factor in the growth and 
distribution of plants, and it deter- 
mines to a great extent the character- 
istic flora of the different regions. 
A good example of a temperature- 
determined agricultural area is the 
Cotton Belt of the South which is a 
region whose mean annual tempera- 
ture is over 60° F. and whose frostless 
season is between 180 and 200 days. 
Another example is the Corn Belt 
where the mean summer temperature 
is always above 65° F. and the average 
night temperature during the 3 sum- 
mer months is never below 55° F. 
Trees in general do not have the 
characteristic short growing seasons 
of the agricultural crops and, con- 
sequently, their growth is a function 
363 
of annual rather than specific seasonal 
temperatures. Consequently, an anal- 
ysis was made of the mean annual 
temperatures of the natural cork oak 
regions with the following results: 
The minimum cardinal growth mean 
annual temperature was found to be 
50° F. in regions where the average 
temperature in January, usually the 
coldest month, was not below 37° F., 
which appears to be the limiting tem- 
perature of Poleward growth. Since 
no cork in the Mediterranean region 
is found beyond these temperature 
limits, it appeared safe to set the Pole- 
ward limit of growth for the potential 
cork areas of the United States as an 
imaginary line whose mean annual 
temperature is not less than 50° F. 
and whose average January tem- 
perature is not below 37° F. Such a 
limiting line is shown in map II (fig. 
2). The region Equatorward of this 
line constitutes the potential cork area 
of the United States. 
The maximum mean annual tem- 
perature at which cork is growing in 
the Mediterranean region was found 
tojbe 70° F. 
A complete analysis of the tem- 
perature distribution of the natural 
cork oak is presented in table 2, which 
shows that the greatest amount (57.2 
percent) of cork is growing in the 
60°-65° F, region. 
TABLE 2.— Temperature distribution of natural cork oak 
Percent in temperature region of— Com- 
posite 
Country temper- 
50°-55° | 55°-60° | 60°-65° | 65°-70°| for) 
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