[Chap. IV SEASONAL ASPECTS OF PLANTS 33 



When the hfe cycle of a plant covers a part of two growing seasons 

 it is called a biennial. Shepherd's-purse may grow both as an annual and 

 as a biennial in regions with mild climates, and wheat is cultivated both 

 as "winter wheat" and as "spring wheat." Special varieties of wheat 

 have been selected for each of these growth regimes. 



During the life cycle of many biennials the first season of growth 

 ends with the formation of a thickened root, a short stem, and a rosette 

 of leaves near the soil surface. These young plants remain dormant dur- 

 ing the winter and in the second growing season complete their life 

 cycle by the development of upright stems, flowers, fruits, and seeds. 

 Then the plant dies. Whether certain plants grow as annuals, biennials, 

 or perennials depends upon the temperature, length of day, and length 

 of the growing season to which they are exposed. 



A B 



Fig. 20. Diagram of winter rosette of leaves of teasel (A), and of evening 



primrose ( B ) . 



The perennial herbs are like annuals and biennials in having a vegeta- 

 tive stage previous to the formation of flowers and fruits, but differ 

 from them in having continuous vegetative growth and seed formation 

 for many successive seasons. The annual active period of growth of 

 perennial herbs is followed by a dormant one when most of the living 

 plant is underground, with no parts extending much above the soil sur- 

 face. Some of these plants have "winter rosettes" (Fig. 20) of leaves, 

 others have very short lateral branches with small leaves, and still 

 others have large buds above ground. Some of the perennials may con- 

 tinue living indefinitely. Perhaps some have been living in their present 



