THE PEA FAMILY. 131 



nodules on the roots. It has been discovered that 

 they always contain minute fungi, generally called 

 microbes (a word meaning " small living beings "). These 

 are enabled in some unknown manner to absorb the nitro- 

 gen gas from the air (which consists of about four-fifths 

 of nitrogen and one-fifth of oxygen, with about 5^- per 

 cent, of carbonic acid gas). No flowering plant is capable 

 hy itself of taking up this beneficial element from the 

 air, but only from mineral ingredients in the soil 

 called " nitrates," as compounds of ammonia (smell- 

 ing salts). The consequence is that leguminous plants 

 are found to contain more nitrogen than any others, 

 so that while, e.g., a potato has only 2 per cent, of 

 " nitrogenous " matters, peas, beans, and especially 

 lentils, have about 24 per cent. Nitrogen must be 

 present in our food for building up the brain, 

 muscles, nerves, bones, and blood, so that no other 

 vegetables are so important in this respect as those 

 derived from the order Legurninosm. 



General Description of the Pea Family. 



Herbs, shrubs, or trees. 



Leaves — Compound, rarely simple, stipulate, some- 

 times as tendrils. 



Floicers — Calyx coherent ; petals iiTegular (except 

 sub-order Mimosem) ; stamens, free, monadelphous or 

 diadelphous ; pistil of one carpel. 



Fruit — A legume. 



