LESSONS IN BOTANY. 219 



the formation of partitions, and the new cells develop into trach- 

 eides or wood cells on the one side and into sieve tissue and bast 

 fibers on the other. The new wood incloses the old, while the old 

 bark is outside the new, serving as a protective coating for it. 



In the class Gymnospermse are included the Cycads, Conifers 

 and the Joint Firs. The conifers are the only ones common in 

 the United States, and the only ones of commercial value. The 

 order conifers includes the juniper, cypress, white cedar, arbor 

 vitce, the giant redwood, the silver fir, the tamarack or American 

 larch, the white pine, the sugar pine of California, the yellow pine 

 of Georgia, and many others. There are some 300 species of 

 pine, growing mostly in cool climates. In general, they are rapid 

 growers, furnishing strong and durable timber and lumber. 



