114 THE NATURE AND WORK OF PLANTS 



and select tlie annuals. What cultivated plants are 

 grown as annuals ? 



155. Biennials. — A second group of species 

 germinate the seeds, form a stem with a rosette of 

 leaves and an extensive root system in one year, and 

 then develop the flower-bearing branches, flowers, 

 and seeds the second season. Some of the thistles 

 and the common mullein belong to this class. 



156. Perennials. — Quite a large number of spe- 

 cies germinate the seeds and form stems the first 

 year, then rest during the winter, and continue 

 growth during the next season and for many succes- 

 sive seasons. In trees and similar forms the entire 

 stem remains alive, and consequently it increases in 

 bulk each year, attaining an enormous size. The 

 giant trees of California have attained a trunk about 

 four hundred and fifty feet high, and thirty feet in 

 diameter. These are by no means the tallest trees 

 in the world. 



Another class of perennials have a stem which lies 

 just underneath the surface of the soil, sending 

 branches up into the air each year. These die down 

 on the approach of winter. Meanwhile the under- 

 ground stem grows at the tip, and would increase its 



