714 



ECOLOGY 



Annuals and biennials.- — The plants hitherto considered may be 

 placed in three general classes : those with uniform seasonal aspect; those 

 in which the leaves are shed at the inception of the period of drought 

 or cold; and those in which all aerial portions are lost at the beginning 

 of the inclement season. The fourth and final class of land plants is 

 that in which the entire plant dies at the inception of the inclement 

 season. The most representative members of this class are the annuals, 







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Fig. 1036. — Winter rosettes of an evening primose (Oenothera), with leaves closely 

 appressed to the ground; note the small amount of leaf overlap, due to high-ranked 

 phyllotaxy and to variation in leaf length; Chicago, 111. — Photograph by Land. 



which are plants that live only in the favorable season, and which have 

 fewer protective structures than do other plants. The annual alone 

 among plants remains through seasons of severity solely in the form of 

 its progeny, the seed. Related to the annuals are the biennials, which 

 are plants that live in two vegetative seasons. In the first season most 

 biennials develop a rosette (as in the evening primrose, peppergrass, and 

 mullein, figs. 1036, 840), which remains as such through the period of 

 drought or cold. During the second vegetative season an erect shoot 

 commonly appears and develops flowers and fruits, death ensuing at the 

 inception of the second inclement period. 



Annuals and biennials do not perennate, because they fail to develop 

 lateral basal shoots ; occasionally, however, some individuals of species 



