i674 A TEXTBOOK OF THEORETICAL BOTANY 



tropical genus with about 350 species. /. leptostachya, I. tinctoria (Fig. 1531) 

 and others are the source of the indigo dyes. The plants are cut just before 

 flowering and are soaked in water producing a yellow solution, which, on 

 exposure to the air, oxidizes and precipitates insoluble indigo. Another 



Fig. 1 53 1. — Indigofera tinctoria. Flowering 

 shoot. 



very important genus is Astragalus, one of the largest in the world. It 

 contains some 1,600 species, the majority of which inhabit the north 

 temperate regions of the Old World, almost all in dry continental areas. 

 About 200 species are found in North America, but none is found in 

 Australia. Many of the species are thorny, the thorns being produced by 

 the petiole or mid-rib which hardens when the lamina disappears. Several 

 possess gums which are used commercially. A. giimniifer is the source of 

 Gum Tragacanth which is obtained by wounding the stem. 



Several genera are grown in gardens. Robinia, with six North American 

 species, is the Locust or False Acacia. The leaflets move upwards in hot 

 air and the base of the petiole forms a cup over the developing axillary buds. 

 Colutea with twelve species occurs from southern Europe to the Himalayas. 

 C. arborescens is the Bladder Senna. Its leaves have the same properties 

 as Senna {Cassia) and are used as an adulterant. The pods are inflated and 

 burst on pressure. Glycyrrhiza is a small tropical genus, important because 

 G. glabra is the source of Spanish liquorice which is extracted from the 

 rhizome and exported chiefly from Iraq. The species Carmichaelia australis 

 is sometimes grown in gardens. It is interesting because it is characteristic- 

 ally xeromorphic, the stems being converted into cladodes without any green 



