[Chap. IV SEASONAL ASPECTS OF PLANTS 35 



appearance of being doiTnant. Roots develop slowly in unfrozen soil, and 

 there may be some transfer of materials within the plant body. In plants 

 with green parts (winter wheat, bluegrass, and evergreen trees and 

 shrubs) food manufacture may occur when daytime temperatures are 

 above the freezing point. Witch hazel and alders may flower during 

 winter thaws, as well as during late autumn and early spring. 



Winter and early spring are the best periods in which to study the 

 characteristic buds, twigs, and bark of the woody perennials. By means 

 of these characteristics one may readily learn to identify trees in winter. 



The leaves of temperate evergreens vary in their endurance of freez- 

 ing temperatures. A sudden exposure to low winter temperatures during 

 midsummer kills them. Most of them, however, withstand temperatures 

 well below the freezing point after they have become "hardened" by 

 exposure to the gradual changes in temperature during autumn. But they 

 do have limits below which injury or death results from low tempera- 

 tures. In some species injury results only when the low temperatures 

 continue for several days. Others are killed by exposure to temperatures 

 below freezing for a few hours. Twigs and parts of larger stems may 

 also be killed by low temperature. In any part of the temperate zone 

 trees and shrubs may be "killed back" by extremely low temperatures. 



The amount of "winter injury" varies from year to year, and many 

 interesting problems occur to the careful observer. Winter injuiy may 

 result from low temperature alone; but in many instances it results 

 from a drying up of the plant during sudden thaws in late winter or in 

 early spring while the soil is still frozen. The roots of winter wheat and 

 other grasses are often broken by the formation of layers of ice beneath 

 the surface of the soil. When subzero temperatures occur, the water in 

 the buds, twigs, and smaller branches is frozen. Still lower temperatures 

 may even freeze the water in the trunks of mature trees. A sudden drop 

 to a very low temperature may result in the splitting of the wood and 

 bark. 



There is another effect of winter that is quite beneficial, or even neces- 

 sary, to many plants. Manv seeds and buds do not germinate readily 

 unless they have been exposed to temperatures near the freezing point 

 for several weeks. Many plants, such as blueberries, unless exposed to a 

 low temperature in the dormant period, do not grow well during the 

 following season. Neither of these cases of "winter conditioning" de- 

 pends upon actual freezing; indeed, many plants grow best after an 

 exposure to a temperature of 5° to 10" above the freezing point for a 



