660 GEOGRAPHICAL DISTRIBUTION OF 



GEOGEAPHICAL DISTKIBUTION OF THE 

 GKAMINE^ OF NOETH AMEEICA. 



Grasses are very widely distributed over the earth's surface. 

 The species are most numerous in tropical regions, where the 

 plants are usually scattered, while in a moist, temperate climate, 

 though the species are less numerous, the number of plants is 

 enormous, often clothing vast areas and open places with a close 

 growth. In temperate regions, where sufficient moisture is wanting 

 to sustain a dense growth, the grasses appear in tufts or bunches 

 more or less isolated. 



Tlie species of grasses of many parts of North America have not 

 yet been sufficiently studied to enable any one to outline with much 

 precision their distribution. This is partly owing to the difficulty 

 of the subject and partly to the lack of thorough exploration in 

 the newer sections, especially in Mexico and countries to the south. 



In Genera Plantarum of Bentham and Hooker the genera of 

 Gramineoe have been recorded at 298; the species, at the highest, 

 about 3200. The number of genera is now known to be a little 

 larger, and the number of species discovered has increased con- 

 siderably. 



Many botanists are inclined to separate grasses into more genera 

 and more species than have the authors of the standard work above 

 mentioned. 



The number of genera native to North America, including the 

 West Indies, so far as discovered and described, is about 140. 



The number of genera introduced, mostly as weeds, 25. 



The number of species native to North America, about 1275. 



The number introduced as weeds, etc., about 105. 



The whole number of genera, 165. 



The whole number of species now known here, 1380. 



No doubt there are still a considerable number especially of 

 native southern species yet to be discovered, and some others will 

 ere long find a home as emigrants from foreign lands. 



