them to grow under different conditions of length of day, tem- 

 perature, and soil moisture; but Stipa spartea and Elymus canaden- 

 sis apparently respond to conditions in widely separated areas not 

 because of genetic differences among the plants of each, but be- 

 cause of the wide range of phenotypic expression of the same 

 genotype. ^'*°-''*^ 



Life-form (Vegetation-form, Hobitat-form, Growth-form) 



By life-form, in a broad sense, is meant the characteristic 

 vegetative appearance such as the size, shape, branching, and at 

 times the histological features of the plant body and its longevity. 

 In a restricted sense it refers to the forms based on the location 

 of the overwintering parts. ^^^ The life-form of a species is caused 

 primarily by its genetic constitution, secondarily by the environ- 

 mental conditions. Pronounced changes in the latter may cause 

 great alterations in life-form. For example, the longevity of many 

 perennial grasses may be reduced, as when rescuegrass {Bromus 

 catharticus), a 4- to 5-year perennial in South America, was intro- 

 duced into the southern United States. Here it became a winter 

 annual, and when introduced into northern latitudes, an an- 

 nual.201 



The life-form influences the economic value of plants in vari- 

 ous ways. For example, strains of grasses having a genetically 

 determined wide leaf to stem ratio and low height, are preferred 

 for pastures because of their greater nutritional value. The Fair- 

 way strain of crested wheatgrass {Agropyron cnstatum), much used 

 in reseeding range land, is smaller, more decumbent, and finer- 

 leaved than the Standard strain. Prostrate forms of legumes are 

 often preferred in pastures because some of the flowers usually 

 escape grazing and produce seeds. These examples illustrate that 

 the selection of suitable life-forms is an important objective in 

 plant breeding. 



All the species in a simple plant community may belong to the 

 same life-form, but most communities have several to many (see 

 Figures 2-5 and 3-1). Grassland stands in the Great Plains include 

 various kinds of life-forms such as perennial rhizomatous mat- 

 formers {Bouteloua gracilis), erect rhizomatous perennials {Agropy- 

 ron smithii), perennial bunchgrasses {Stipa comata), succulent 



92 • THe Comnmimnity 



