Fixation of Nitrogen.Another, and perhaps an even more important, instance of symbiotic association has come to the front during the same period, it is an alliance between the plants of the Natural Order Leguminosae and certain bacterium-like forms which find a home within the tissues of their roots.
Many writers in recent years, among whom may be named especially Heliriegel and Wilfarth, Lawes and Gilbert, and Schlcesing and Laurent, have shown that the Leguminosae as a group form conspicuous exceptions to this rule.
The power exercised by the Leguminosae is associated with the presence of curious tubercular swellings upon their roots, which are developed at a very early age, as they are cultivated in ordinary soil.
In India proper, with a dryer climate, grasses and Leguminosae take the lead in the number of species.
Amongst arboreous families Leguminosae and Euphorbiaceae are prominent; Hevea belonging to the latter is widely distributed in various species in the Amazon basin, and yields Para and other kinds of rubber.