LIGHT AND HEAT 75 



Its protoplasm must be made of sterner stuff than most 

 plants. 



Although little is known as to the physiological causes 

 which enable some plants to withstand low temperatures, 

 it is probable that the density and nature of the sap play 

 an important part. It has been demonstrated that the 

 most hardy of the magnolias possess the densest sap. 

 Again, it has been shown in the case of some plants which 

 resist severe frosts that the insoluble starch in the leaves 

 becomes replaced by soluble sugar during the winter, 

 whilst seedlings and leaves which have been fed with a 

 sugar solution can withstand a lower temperature than 

 they normally do. The reason that has been suggested 

 for this is that, in the ordinary course, when the sap 

 begins to freeze, water is withdrawn and the sap becomes 

 more concentrated, with the result that the proteins in 

 solution in the sap and protoplasm separate out, but that 

 if sugar is present this ' salting out " process takes place 

 at a much lower temperature. 



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. 



