66 



FOUNDATIONS OF BIOLOGY 



by the SECONDARY roots which radiate and branch in all direc- 

 tions from the main axis of the plant until they form a com- 

 plex underground structure. This may equal in size the 

 part of the plant body which is developed above the surface 

 by the shoot system. 



In plants which live through two years (BIENNIALS) , often 



Spring Seedling 



germination growth 



Seed (winter rest) 



Adult plant 



FIG. 27. The seasonal history of an annual plant, a Bean. 

 (From Densmore.) 



the first year is spent in storing up food. Sometimes this is 

 in the roots, in which case they are greatly enlarged to form 

 a reservoir of material, at the expense of which during the 

 second season the plant rapidly develops flowers and seeds. 

 These storage roots may be tap roots as in the Turnip, or 

 lateral roots as in the Dahlia and Sweet Potato. (Figs. 26, 27, 28.) 

 Although the contact of the plant with its environment 

 through its roots is ordinarily underground, tropical plants 

 in particular frequently develop AERIAL roots from the stem or 



