96 Science of Plant Life 



life is passed within a twelvemonth period, these plants are 

 called biennials, because their life period covers parts of two 

 vegetative seasons. 



The term annual or biennial as applied to plants, therefore, 

 does not imply any definite length of life in months. Wheat 

 may be grown either as an annual or as a biennial, depending 

 upon whether it is planted in the spring or in the fall. 1 Shep- 

 herd's purse and wild lettuce not infrequently live as annuals 

 in nature. The commonest biennials of the garden are beets, 

 carrots, parsnips, turnips, and cabbage. In the first four, 

 large amounts of food are accumulated in the roots; in the 

 cabbage the food is stored in the enormous terminal bud, the 

 " head." These stores of food are used in the production of 

 seeds the following year. 



Usually biennials and annuals are herbs. Biennials, like 

 annuals, are comparatively small in size, and die after flowers 

 and seeds have been produced. 



Perennials. Perennials (Latin : perennis, lasting through 

 the year) are plants that live for a number of years. Some of 

 them, as for example certain grasses, produce seed during the 

 first and succeeding years. Other perennials, like alfalfa, form 

 seed at the end of the second and succeeding seasons. Trees 

 and shrubs usually require several seasons' growth before 

 seeds are produced. The century plant of our southwestern 

 deserts develops vegetatively for 25 or 30 years before it pro- 

 duces a flowering stem and seeds. Then it behaves like an 

 annual or a biennial, for as soon as the seeds are mature the 

 whole plant dies. This calls our attention to the interesting 



1 When wheat is planted in the spring and grown as an annual, it is called 

 " spring wheat." When it is planted in the fall and grown as a biennial, it is 

 called " winter wheat." The same variety may be grown either as spring 01 

 winter wheat, but in practice different varieties are grown. 



