[Chap. L THE SEED PLANTS 679 



of the seed plants are herbs, commonly less than a few feet in height and 

 rarely over 25 feet. The smallest seed plant known is a floating duck- 

 weed (Wolffia) scarcely more than l/20th of an inch in diameter. 



Many seed plants complete their life cycles in a single season, others 

 in 2 to 30 seasons; the remainder flower annually over a long period of 



Fig. 320. A large American elm at Lancaster, Massachusetts. 



years. The oldest living things on earth are trees. The Big Cypress 

 Tree of Tule ( Fig. 319 ) and the giant sequoias of California have lived 

 for 3000 to 4000 years. Elms (Fig. 320) and maples rarely live longer 

 than 300 years, oaks 500 years, and Douglas fir and western arbor vitae 

 700 to 800 years. 



