AGENTS IN THE DISSEMINATION OP PLANTS. 567 



others are confined within narrow limits. Some of the common weeds 

 in Britain, such as Chickweed, Shepherd's- purse, and Groundsel, are 

 found at the southern extremity of South America. Lemna minor and 

 trisulca, Convolvulus sepium, Phragmites communis, Cladium Maris- 

 cus, Scirpus lacustris, Juncus effusus, and Solanum nigrum, are stated 

 by Meyen to be common to Great Britain and New Holland. Nas- 

 turtium officinale, and Samolus Valerandi, are very extensively diffused, 

 and they may be reckoned true cosmopolites. They are both natives of 

 Europe, and they occur, the former near Rio Janeiro, the latter at 

 St. Vincent. The lower the degree of development, the greater seems 

 to be the range. Some Cryptogamic plants, as Lecanora subfusca, are 

 found all over the world. Man has been instrumental in diffusing 

 widely culinary vegetables, such as the potato, and the cereal grains, 

 as well as many other plants useful for food and manufacture. Corn 

 plants, such as Barley, Oats, Rye, Wheat, Spelt, Rice, Maize, and Millet, 

 are so generally cultivated over the globe, that almost all trace is lost of 

 their native country. They can arrive at perfection in a great variety 

 of circumstances, and they have thus probably a wider geographical 

 range than any other kind of plant. As regards these plants, the 

 globe may be divided into five grand regions the region of Rice, 

 which may be said to support the greatest number of the human race ; 

 the region of Maize; of Wheat; of Rye; and lastly, of Barley and Oats. 

 The first three are the most extensive, and Maize has the greatest 

 range of temperature. The grains extending farthest north in Europe 

 are Barley and Oats. Rye is the next, and is the prevailing grain in 

 Sweden and Norway, and all the lands bordering on the Baltic, the 

 north of Germany, and part of Siberia. Wheat follows Rye; it is 

 cultivated in the middle and south of France, England, part of Scot- 

 land, part of Germany, Hungary, Crimea, and the Caucasus. We next 

 come to a district where wheat still abounds, but no longer exclusively 

 furnishes bread, rice and maize becoming frequent. To this zone 

 belong Portugal, Spain, part of France, Italy and Greece, Persia, Nor- 

 thern India, Arabia, Egypt, the Canary islands, &c. Wheat can be 

 reared wherever the mean temperature of the whole year is not under 

 37 or 39 F., and the mean summer heat, for a period of at least three 

 or four months, is above 55. It succeeds best on the limits of the 

 subtropical region. In the Scandinavian Peninsula, the cultivation of 

 Barley extends to 70 north latitude, Rye to 67, and Oats to 65. 

 The cultivation of Rice prevails in Eastern and Southern Asia, and it 

 is a common article of subsistence in various countries bordering on 

 the Mediterranean. Maize succeeds best in the hottest and dampest 

 parts of tropical climates. It may be reared as far as 40 north and 

 south latitude on the American continent on the western side, while 

 in Europe it can grow even to 50 or 52 of latitude. It is now cul- 

 tivated hi all regions in the tropical and temperate zones, which are 



