412 LEGUMINOS^ 



Through symbiosis with a bacterium which penetrates the 

 roots, the Legurainosse are able to thrive upon ground which 

 is devoid of combined nitrogen : the nitrogen which they require 

 for growth is obtained indirectly from the free nitrogen of the air 

 (see p. 793). 



Usually a cereal and especially wheat is taken after the growth 

 of a leguminous crop. 



Some species, such as vetches and lupins, are occasionally 

 grown on poor, dry ground to be subsequently ploughed in as a 

 ' green manure ' ; this practice largely increases the nitrogen- 

 content of the soil and at the same time augments the stock 

 of humus in the latter. 



3. The genera most important from a farmer's point of view 

 are the following : — 



Pisum (peas), Vicia (vetches and common bean), Trifolium 

 (the true clovers), Medicago (the medicks — ^lucerne and yellow 

 trefoil), Onobrychis (sainfoin), Anthyllis (kidney-vetch), and Lotus 

 (birds'-foot trefoil). 



Some common plants of less importance belonging to other 

 genera are Gorse or Whin (genus Ulex), Bokhara clover (genus 

 Melilotus), Everlasting pea (genus Lathyrus), Lupins (genus 

 Lupinus) ; and in gardens Scarlet Runner and Dwarf Kidney 

 Beans of the genus Phaseolus. 



4. Peas (genus Pisum). — The cultivated varieties of peas are 

 usually supposed to belong to two species, namely : (1) the 

 Field Pea {Pisum arvense L.), which is said to be found in a 

 wild state in the south of Europe, and (2) the Gkirden Pea 

 {Pisum sativum L.), which is not known wild, and may possibly 

 be a modified form of the former species. 



The Garden Peas, of which there are endless varieties, have 

 white flowers, and seeds of uniform yellowish white or bluish 

 green colour: they are also more delicate and suffer more 

 readUy from frost and drought than the field pea. 



Some of the garden forms for human consumption are grown 



