
THE STEM 161 
probably averages from 28° to 30°C. If the temperature rises 
beyond the optimum, the rate of growth decreases until a tem- 
perature is reached above which no growth takes place. This 
temperature is known as the maximum. The minimum and the 
optimum temperatures for growth vary not only with different 
species but also with changes in other environmental conditions. 
The effect of tem- 
perature is not con- 
fined to the direct 
action of favorable or 
unfavorable tempera- 
ture on the rate of 
erowth. Thus, trees 
from the temperate 
zone do not thrive 
in the tropics, where 
the temperatures are 
favorable to growth 
throughout the year. 
Trees which are na- 
tive to the temperate 
zone seem to need a 
winter season, while [F1c.158. Photographs of blueberry twigs, both 
tropical ones do not taken in the spring at the same time 


Trees and shrubs Theoneon the left had been exposed toa cold winter 
while the other had been kept warm in a greenhouse 
of the temperate zone, (After Coville) 
which are usually 
dormant during the long winter, tend to become dormant in the 
tropics after a period of activity. Unless they are exposed to an 
extensive period of cold though not necessarily freezing weather, 
the resumption of growth is greatly delayed, while the growth 
is lacking in vigor (Fig. 158). It is probably for this reason 
that such fruits of the temperate zone as apples, pears, peaches, 
and cherries do not thrive in the tropics. They grow well for 
a time and then become half dormant and, in the absence of 
chillme, never fully recover from this condition. 

