LIGHT AND HEAT 75 
Its protoplasm must be made of sterner stuff than most 
plants. 
Acclimatization of Plants.—Southern races habituated 
to warm conditions are very susceptible to cold, and not 
easily cultivated in cold countries, except in warm green- 
houses. Some plants, however, may, if suitable measures 
be taken, be gradually acclimatized. During the winter 
they are kept in the greenhouse ; in the spring they are 
not put out in the open at once, but are first transferred 
to a cooler house or frame till they are hardened off. 
In this way they are gradually inured to colder conditions 
before being bedded out. 
