196 THE STORY OF PLANT LIFE 



the British species being included in the Papi- 

 lionaceae. 



In the Mimosoideae the flowers are regular and the 

 corolla is valvate. They include such plants as the 

 Acacia, Mimosa, etc. The Csesalpinioidese include 

 those plants in which the flowers are zygomorphic, 

 the corolla being imbricate and ascending. In the 

 Papilionatse the flowers are zygomorphic, papi- 

 lionaceous, the corolla imbricate, descending. In 

 the Csesalpinoideae are included Gleditschiaj Cassia, 

 Tamarindiis, and Copaiba, all useful plants. Amongst 

 Papilionatae Onobrychis has a lomentum, in the rest 

 the fruit is a legume or pod. 



The Leguminosae inhabit all countries, are found 

 in all manner of habitats, and include all sorts of 

 plants, differing in habit from the prostrate, her- 

 baceous Bird's Foot, to the graceful trees. Acacias, 

 often planted in this country. Some are aquatic, 

 and many are adapted to dry soil conditions. A 

 large proportion are climbers, as the Vetches, Vicia, 

 and Lathy rus. 



One character that they possess is of special 

 interest, namely the power they have of fixing 

 nitrogen, and so increasing soil fertility. This is due 

 to the agency of nitrogen-fixing bacteria or lowly 

 micro-fungi, which develop in tubercles or altered 

 lateral shoots of a gall-like nature. It is the free 

 nitrogen of the atmosphere that is absorbed by 

 these bacteroids ; hence their presence is a distinct 

 advantage, adding to the nitrates derived from 



