Dir, 
* THE STEM 163 

which would be killed by the first heavy freeze. These evil 
results are prevented when two or three months of chilling are 
necessary before a dormant plant will respond to a favorable 
temperature by the resumption of growth. 
Light. As the growth of green plants is dependent on photo- 
synthesis, plants cannot continue to grow for any great length of 
time without light. On the other hand, high light intensities 
are usually accompanied by rapid rates of evaporation. In the 
preceding chapter it was pointed 
out that plants may contain less 
water on sunny days than on 
cloudy days or during the night. 
This condition frequently re- 
sults in amuch more rapid rate of 
growth during the night than 
during the day, and plants may 
even cease growing or actually 
shrink during bright, dry days. 
The relative length of day 
and night is a very important 
factor in determining the fruiting Fic. 160. Fasciated stem of 
season of plants. Some plants, as auckscomb 
poinsettia (Fig. 80), will flower 
and fruit only during seasons with short days, and others only 
when the days are long. In temperate zones the days are long 
during the summer, shorter in autumn, and still shorter in winter. 
It has been found that many plants which normally do not flower 
until the short days of autumn can be made to bloom during the 
long days of summer if they are placed in the dark for a portion 
of each day (Fig. 159). Also, many plants that usually blossom 
in the summer and not in the winter, even if in a heated green- 
house, will produce flowers during the short days of winter if 
kept in a warm greenhouse and illuminated by electric lights for 
a portion of each night (Fig. 159). 
While the period of flowering may be hastened by shorten- 
ing the daily exposure to light, the rate of growth frequently 


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