668 AGENTS IN THE DISSEMINATION OF PLANTS. 



II. — Dissemination op Plants. 



1. — ^Agents employed in their Dissemination. 



Some plants are disseminated generally over the globe, whUe 

 others are confined within narrow limits. De OandoUe says that no 

 phanerogamous plant is a cosmopolite in an absolute sense. Some 

 extend over more than one-third of the earth's surface, but none ap- 

 pear to compass the whole earth. 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 

 Mariscus, Scirpus lacustris, Juncus eflfusus, and Solanum nigrum, are 

 stated by Meyen to be common to Great Britain and Australia. 

 Nasturtium officinale, and Samolus Valerandi, are very extensively 

 diffused, and they may be almost reckoned cosmopolites. They are 

 both natives of Europe, and they occur, the former near Kio Janeiro, 

 the latter at St. Vincent. Trisetum subspicatum is a grass having a 

 range from Tierra del Puego to Greenland ; Drimys Winteri, Winter's 

 bark, extends in South America over 5000 geographical miles. Many 

 European plants are found in the antarctic regions. Potentilla 

 anserina, EpUobium tetragonum, Oxalis corniculata, Hymenophyllum 

 WUsoni, Galium Aparine, Urtica dioica, Chenopodium album, and 

 Cynodon Dactylon, are very widely distributed. The lower the" degree 

 of development the greater seems to be the range. Some Crypto- 

 gamic 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, 

 Eye, Wheat, Spelt, Kice, Maize, and Millet, are so generally culti- 

 vated over the globe that almost all trace is lost of their native 

 country. They can arrive at perfection in a great variety of circum- 

 stances, 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 regions — the region of Eice, which may be said to 

 support the greatest number of the hrmian race ; the region of 

 Maize ; of Wheat ; of Eye ; 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. Eye is the next, and is the prevailing 

 grain in Sweden and Norway, and aU the lands bordering on the 

 Baltic, the north of Germany, and part of Siberia. Wheat follows 



