THE GTMNOSPERM TREES 



257 



of individual forest products. At the same time, however, pure 

 stands invite destruction by fire, insects and fungi more readily 

 than do mixtures of tree species. 



Conifers have a habit of growth known as excurrent (fig. 1 80) ; 

 stem growth is all concentrated in one main axis which charac- 

 teristically forms a straight, undivided central shaft about which 

 are placed the branches. This is in contrast to the deliquescent 

 form exhibited by most broadleaved trees, in which the main 







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A B 



Fig. 180.-^Trees have two types of branching, excurrent (A) and deli- 

 quescent (B) ; the former is typical of many conifers while the latter is char- 

 acteristic of most deciduous trees. 



stem is often divided several times. The concentration of wood 

 in one central stem makes for greater ease of sawing and higher 

 yield of lumber, since the trunks are straight and free from large 

 knots; this is a further reason for the great importance of conifers 

 over broadleaved trees as timber producers. 



The various members of this group are spoken of interchange- 

 ably as "conifers," "softwoods," or "evergreens." The first 

 name applies to the seed-bearing structure (cone) found in three 

 of the four families native to North America. The term "soft- 

 wood" is commonly used by lumbermen to distinguish between 

 conifers and broad-leaved trees or "hardwoods." Very often, 

 however, the wood of a "hardwood" (basswood or poplar) is 



