1670 A TEXTBOOK OF THEORETICAL BOTANY 



In such a vast assemblage of genera as are included in the Papilionaceae 

 it is obviously impossible to do more than touch quite briefly upon a few of 

 the more outstanding. In the Sophoroideae the more important genera, 

 Sophora and Ormosia, are widely distributed throughout the tropics, though 

 the sub-family also includes a number of small or monotypic genera mostly 

 restricted to the warmer parts of America. Species of Sophora are culti- 

 vated in the warmer parts of Britain, where their large yellow flowers make 

 them desirable shrubs. The wood is very hard. S.japonica is the source of 

 the dye which was used for the Imperial yellow of China. 



The sub-family Podalyrioideae contains some forty genera, the great 

 majority of which are restricted to Australia; two genera occur in South 

 Africa the remainder in temperate Asia and North America. They are of 

 little economic or cultural importance. 



The third sub-family, Genistoideae, contains a number of important 

 genera, several of which occur wild in Britain. Species of Crotalaria and 

 more particularly C. jiincea and C. retusa are large annual plants which are 

 cultivated for the fibres which are obtained by maceration of the stem. 

 These fibres form the Sunn Hemp or Madras Hemp which is exported 

 from India as a cheap substitute for European Hemp. Crotalaria is a large 

 genus of some 350 species w^hich is widely distributed in the tropics and 



subtropics. 



The genus Lupiniis, from which the garden Lupins (L. polyphyllus) 

 have been bred, is principally American, with about 150 species. The fruit 

 is explosive, the two valves of the legume twisting spirally to eject the seeds. 

 In some countries, species of Liipinus, chiefly L. liiteiis, are grown as a field 

 crop like clover, the stems being used as fodder and the seeds as an article 

 of food in south Europe. 



Laburnum, a tree, with three species, and Cytisus, a shrub, with forty 

 species, are both natives of southern Europe. In the latter genus the leaves 

 are reduced or scaly and assimilation is performed by the green stems. The 

 fruit explodes as in Lupinus. The first "graft hybrid", one between the 

 Common Laburnum (L. anagyroides) and the purple Broom {Cytisus 

 purpureiis), was produced by J. L. Adam in his nursery at Vitry near Paris 

 in 1825. This was a graft of the Broom on Laburnum which produced 

 shoots of an intermediate type. Older trees showed branches not only of 

 the hybrid but of both parents as well. These graft hybrids are termed 

 chimaeras, because there seems to be a mixture of the tissues of both 

 parents, rather than a genetic fusion. On microscopic examination it is 

 often found that the outer tissues are characteristic of one parent, which 

 seem to form a covering over the inner tissues which are characteristic of 

 the other parent. We shall refer again to graft hybrids in Volume HI. 



In the genera Ulex (Gorse) and Cytisus (Broom) the seeds possess at 

 one end, near the hilum, an orange-coloured appendage which is termed a 

 caruncle or elaeiosome. It is rich in oil which is apparently attractive 

 to ants, which carry ofi^ the seeds, thereby assisting in their distribution. 

 The pollination mechanism in this sub-family is explosive. There are 



