The great plains are covered with edible grasses, divided into two classes, pasto duro (hard grass) and pasto blando, or tierno (soft grass) - the former tall, coarse, nutritious and suitable for horses and cattle, and the latter tender grasses and herbs, including clovers, suitable for sheep and cattle.
To the former belong the ordinary leguminous crops - the clovers, beans, peas, vetches or tares, sainfoin, lucerne, for example - which obtain their nitrogen from the air, and are independent of the application of nitrogenous manures, whilst in their roots they accumulate a store of nitrogen which will ultimately become available for future crops of other kinds.
Weeds, therefore, which need sour conditions for development are checked by liming and the better grasses and clovers are encouraged.
The Seed Control Act of 1905 brings under strict regulations the trade in agricultural seeds, prohibiting the sale for seeding of cereals, grasses, clovers or forage plants unless free from weeds specified, and imposing severe penalties for infringements.
In his book on the fertilization of flowers, Hermann Muller distinguishes four types of papilionaceous flowers according to the way in which the pollen is applied to the bee: (I) Those in which the stamens and stigma return within the carina and thus admit of repeated visits, such are the clovers, Melilotus and laburnum.