THE DICOTYLEDONES 1663 



juliflora, the Mesquite Tree of the Central American savannahs, the 

 branches of which are used for fodder. Albizzia moluccana is frequently 

 used as a shade tree in young tea plantations on account of its very rapid 

 growth, reaching 10 ft. in a year. 



Finally we may mention the genus Mimosa itself, which is a large tropical 

 and subtropical genus with about 400 species, most of which occur in 

 America in contrast to the Australian Acacias. They are mainly herbs or 

 small shrubs which are frequently beset with stipular spines. The best- 

 known example is the pan-tropical Mimosa piidica, called the Sensitive 

 Plant on account of the movement of its leaves when disturbed (see Volume 

 III). 



Papilionaceae (Viciaceae) 



The Papilionaceae are a very large family which includes those plants 

 which are sometimes referred to as the Pulses or, in common parlance, the 

 Peas and Beans. These plants are biologically remarkable in that they 

 possess the power of utilizing atmospheric nitrogen, through the agency of 

 the bacteria which occur in nodules in their roots (see Volume I). In 

 consequence they are of first-rate importance in agriculture. Not only 

 are the seeds rich in food reserves and therefore valuable for food, but at 

 the same time the aerial parts are useful as cattle fodder. After these have 

 been cut and gathered, the roots left behind liberate nitrates as they decay 

 and so improve the soil. 



In temperate climates these leguminous plants are generally grown as 

 part of a crop rotation, immediately preceding cereals such as Wheat, or 

 other crops with a high nitrogen demand. In Britain the Red Clover is 

 usually included in the four-year rotation; roots, Barley, Clover, Wheat. In 

 tropical countries leguminous crops frequently serve an additional function. 

 Due to the intensity of the sun, bare ground loses water from the surface 

 very rapidly and freshly planted crops would very probably die before they 

 could establish themselves. It is customary, therefore, to plant what is 

 termed a Cover Crop, generally a legume, which covers the ground around 

 the young plants and helps to protect them. In some instances a further 

 cover crop of quick-growing trees, such as the leguminous Erythriua, may 

 also be planted to serve the same purpose. As the crop develops the legu- 

 minous cover crop is cut and mulched, whereby the plants of the economic 

 crop receive additional nourishment. If this economic crop consists of 

 perennials, as for example Coffee or Bananas, a tree cover crop will be 

 planted and eventually cut down, when the Bananas are sufficiently grown. 

 On the other hand a herbaceous cover crop may be planted time after 

 time to cover the ground and prevent not only loss of water but also the 

 encroachment of weeds which might eventually stifle the main crop. 



Among species which are either wild or commonly grown, we may 

 mention the Garden Pea (Pisiim) (Fig. 1524), the Beans {Vicia and 

 Phaseolus), the Vetches and Alfalfa (Vicia), the Clovers (Trifolium), Lucerne 

 [Medicago sativa), and Sainfoin (Onobrychis). A number of common shrubs 



