CHAPTER VIII 



THE PLANT IN RELATION TO TEMPERATURE, LIGHT, AND 

 MOISTURE 



Growth temperature. The range of temperature that vege- 

 tation in the aggregate can endure is remarkable. Seeds in 

 the dormant condition have been exposed to the temperature 

 of liquid air, many degrees below zero, without impairing 

 their vitality ; and, on the other hand, some algse can exist in 

 hot springs where the temperature of the water reaches nearly 

 to the boiling point. No single species, however, can endure 

 anything like this range of temperature. Ordinary plants are 

 balanced midway between two rather close extremes of heat 

 and cold, growing Well so long as neither is too closely 

 approached, going into a dormant condition when brought 

 nearer, and dying when either extreme is reached. Differ- 

 ences in temperature are among the principal factors control- 

 ling the distribution of plants. Elevated country and mountain 

 ranges act as barriers to the spread of tropical plants, because 

 the upper regions are cold, and a stretch of warm lowland may 

 prevent the migration of alpine vegetation from one summit 

 to another; in fact, there is scarcely a species that is not 

 sharply limited in some part of its range by temperature. 



A temperature of 122 above zero is fatal to most land 

 plants in the growing condition, and aquatics usually perish 

 at somewhat lower temperatures. Plants and plant parts gen- 

 erally can endure the greatest amounts of heat and cold when 

 they contain the least water. In seeds, developed by the plants 

 for carrying them over unfavorable seasons, the protoplasm 

 is brought to the resting and more resistant condition by the 



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