DURATION OF ROOTS 



139 



rangement of phloem and xylem is called the radial 

 arrangement. The radial arrangement is pecuKar to roots. 

 It does not occur in the stems. 



44. Duration of Roots. Annuals, Biennials, and Peren- 

 nials. — The length of the Hf e of a plant depends princi- 

 pally upon the length of the life of its roots. Upon the 

 basis of their length of life plants are classified as annual, 

 biennial, and perennial. An annual is a plant which com- 

 pletes its life in one growing season. Thus a plant which 

 begins its growth froiii the seed in spring and dies, root 

 and all, when winter comes, is an annual. A biennial is a 

 plant which Hves in two growing seasons, while perennials 

 are plants which live on from season to season indefinitely. 



Trees and woody shrubs are perennials, and there are 

 many other perennials which are aHve in the winter though 

 we do not see them. They are plants whose parts above- 

 ground die down just as annuals die down. But their 

 underground parts continue to Kve, even in frozen soil. 

 Dandelion and grass are very common examples of this 

 sort of perennial, and most of the flowers which you find 

 in the woods in early spring are the flowers of perennials 

 of this sort. 



Some plants are either annual or biennial. Wheat is 

 an example. Winter wheat is wheat which is sowed in the 

 fall. It sprouts and the whole field may be green with 

 it before the snow falls. But it is a hardy plant. The 

 snow blanket protects it and it Kves on through freezing 

 weather. In the spring it starts to grow again, and the 

 crop is ready for harvest in early summer. Since this 

 winter wheat Hves in two growing seasons, it is a biennial. 

 Spring wheat, however, is an annual. It is sowed in the 



