Longevity of Herbaceous and Woody Stems 145 



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. Shepherd's purse and wild lettuce not in- 

 frequently 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 1? 



stores of food are used in the production of seeds cj^ 



the following year. Usually biennials and annuals i^^*^ 



are herbs. Biennials, like annuals, are compara- 

 tively small in size, and die after flowers and ^^ 

 seeds have been produced. 



Perennials. Perennials (Latin : perennis, last- 

 ing 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 



1 r .^ 1 r\ Fig. 84. Moth mullein, a biennial : first- 



years before it produces a flower- ^^ason rosettes (in foreground) and the 



ing stem and seeds. Then it be- mature plant, 

 haves 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 fact that in annuals, biennials, and a few perennials 

 there is no well-marked period of senility or old age. They die 

 suddenly at maturity, immediately after their period of greatest 

 vigor. Trees and shrubs, on the contrary, have a distinct 

 period of old age in which the physiological processes are slowed 



