HOW TREES GROW AND REPRODUCE 



57 



There are some notable exceptions to this classification because 

 some softwoods as larch and cypress are deciduous and drop their 

 leaves in the autumn. Some hardwoods, such as live-oak, holly, 

 cucumber, etc., are evergreen and retain their green foliage through 

 the winter season. Some of the hardwoods produce wood that is much 

 softer than some of the soft- 

 woods, and vice versa, although 

 generally the hardwoods pro- 

 duce wood that is much heavier, 

 harder, stronger, and more dur- 

 able than the softwoods. The 

 softwoods are chiefly used for 

 construction purposes and form 

 about four-fifths of all our tim- 

 ber supplies. The, hardwoods 

 are used for much more special- 

 ized purposes as furniture, auto- 

 mobile parts, tools, handles, 

 machinery, flooring, etc. 



5. HOW TREES GROW AND 

 REPRODUCE 



Trees grow in height and di- 

 ameter, and therefore in volume, 

 by adding an annual layer of 

 wood. Trees are actually made 

 up of successive cones, each rep- 

 resenting a year's growth, and 

 one being laid literally on top 



of and outside the other. Annual layers of wood are sharply 

 defined in temperate and northerly climates, whereas there is no defi- 

 nition in tropical climates because trees grow throughout the year. 

 A large portion of growth under our conditions is generally added 

 during the first few months of the growing season. During cold 

 weather, trees are dormant. The growing season may be only from 2 

 to 3 months in duration or less in northern climates and at high alti- 

 tudes, whereas it may be from 5 to 10 months or more in the warmer 

 climates and at the lower elevations. 



There is a definite relationship between altitude and longitude. 

 For example, Douglas fir is found at low altitudes along the coast of 

 British Columbia whereas it is found only at the highest elevations 

 at the extreme upper limits of timber growth near timber line in the 



FIG. 30. Bark, cones, needles and pollen 

 cones of longleaf pine (Pinus palustris). 



