V. _Plants belonging to alluvial and marshy soils._--Reed, _Arundo phragmites_, _Poa aquatica_, _Poa fluitans_. Rush, _Juncus conglomeratus_ (order _Juncaceæ_).
After these different soils have been brought under cultivation, the characteristic species, which we have just enumerated, disappear, and are replaced by other plants, which grow, to all appearance, spontaneously, under the name of _weeds_; but, in reality, spring from germs or seeds too frequently mixed up with the different manures, or spread abroad by the agency of birds or the wind.
In reference to this latter consideration, the diffusion of plants, we shall transcribe an interesting passage from Balfour's excellent "Manual of Botany."
"Some plants," he remarks, "are disseminated generally over the globe, while 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. _Laura minor_ and _trisulca_, _Convolvulus sepium_, _Phragmites communis_, _Cedium Mariscus_, _Scirpus lacustris_, _Juncus effusus_, and _Solanum nigrum_, are said to be common to Great Britain and New Holland. _Nasturtium 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 globe.
"Man has been instrumental in widely distributing 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 plants.
"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.