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TEXTBOOK OF BOTANY 



winter temperatures, by the shortness of the growing season which 

 prevents the maturing of the vegetative branches and seeds, and by the 

 presence of species of the hemlock-hardwood and white spruce forests. 

 Southward the characteristic species of the deciduous forest are re- 

 stricted by the absence of a period of low-temperature dormancy, neces- 

 sarv both for the renewal of growth of the buds and cambiums of the 

 perennial plants and for the after-ripening of seeds. Leached sandy soils 

 and tlie absence of humus due to recurrent fires on the coastal plain 



Fig. 384. Eastern white pine. An original stand at Cook Forest. Pennsyhania. 



and mountain tops make conditions more favorable to the southern pines 

 and to other coastal plain species. Far south on the coastal plain, sur- 

 rounded by pine forests, are outlying communities of the deciduous 

 forest called "hammocks." The soils where they occur are better than 

 those of the general region, and the sites have been protected from fires. 

 These communities consist of either mixed oak and hickory or beech- 

 magnolia, with an undergrowth containing many typical deciduous forest 

 shrubs and herbs. 



In many parts of the deciduous forest region are local communities 

 which are relicts of communities that lived under various former cli- 

 mates. Scattered bogs dating back to early postglacial time are still cov- 

 ered with mosses and flowering plants characteristic of the boreal and 



