For many years clover was regarded as a crop helpful to the soil, and one reason given was the great length of the roots. Thus the roots of red clover often reach the depth of several feet, even in heavy soil, which they thus aerate and drain, especially when they decay and leave channels. But this is only half the story; for a long time people had noted that on clover roots were little swollen places or nodules, which were supposed to have come from some disease or insect injury. The scientists became interested in the supposed disease, and they finally ascertained that these nodules are filled with bacteria, which are the underground partners of the clovers and other legumes. These bacteria are able to fix the free nitrogen of the air, and make it available for plant-food. As nitrogen is the most expensive of the fertilizers, any agency which can extract it from the free air for the use of plants is indeed a valuable aid to the farmer. Thus it is that in the modern agriculture, clover or some other legume is put on the land once in three or four years in the regular rotation of crops, and it brings back to the soil the nitrogen which other crops have exhausted. An interesting fact about the partnership between the root bacteria and the clover-like plants is that the plants do not flourish without this partnership, and investigators have devised a method by which these bacteria may be scattered in the soil on which some kinds of clover are to be planted, and thus aid in growing a crop. This method is to-day being used for the introduction of alfalfa here in New York State. But the use of clover as a fertilizer is not limited to its root factory for capturing nitrogen; its leaves break down quickly and readily yield the rich food material of which they are composed, so that the farmer who plows under his second-crop clover instead of harvesting it, adds greatly to the fertility of his farm. The members of three distinct genera are popularly called clovers: The True Clovers (_Trifolium_), of which six or seven species are found in New York State, and more than sixty species are found in the United States. The Medics (_Medicago_), of which four species are found here. The Melilots (_Melilotus_), or sweet clovers, of which we have two species. THE TRUE CLOVERS. (_Trifolium._) [Illustration: _Fig. 245. The common red clover._]