ORDER OXALIDEJ3, OR WOOD-SORREL TRIBE. 413 



acid is peculiar in its properties, and is named oxalic from its 

 existing in this plant, from which, indeed, it was at first ob- 

 tained. Its uses in the arts have already been noticed (. 401). 

 The quantity obtainable from this and other plants which furnish 

 it, is too small for the supply of these; since, from twenty 

 pounds of the leaves of the Wood Sorrel, which yield about six 

 pounds of juice, not quite three ounces of oxalate of potash can 

 be obtained. Since, therefore, it has been ascertained that the 

 acid might be obtained pure by the action of nitric acid upon 

 sugar, this mode of producing it has been generally adopted, 

 except in places where the plant is very abundant. One Indian 

 subdivision of this tribe differs from the rest, in the larger size 

 of the species belonging to it, which grow into bushes or even 

 trees; and some of these are cultivated for the sake of the 

 juices afforded by the leaves, flowers, and fruit, which are of a 

 pleasant acid character, and are esteemed as cooling remedies in 

 fevers. One species, moreover, is remarkable for the great irri- 

 tability of its leaves, which perform movements resembling those 

 of the Sensitive Plant ; and there is a foreign species of Oxalis, 

 that has in some degree the same property. 



578. The last order of Thalamifloral Exogens to be here 

 noticed is that of RUTACE^E, the Rite tribe, of which several 

 species are cultivated in our gardens, though none are originally 

 natives of this country. All the plants of the order are remark- 

 able for their powerful odour, which is usually of a nauseous 

 character ; and this is due to the quantity of essential oil, secreted 

 in little cavities beneath the cuticle of the leaves, the place of 

 which, as in the Orange tribe, is marked by half-transparent dots. 

 To this group belongs the Fraxinella, formerly mentioned (. 374) 

 as rendering the surrounding air inflammable in warm weather, 

 by the quantity of this oil which it diffuses through it. The com- 

 mon Garden Rue will give a very good idea of the structure 

 characteristic of the order. The parts of its flower, however, are 

 arranged in fours ; whilst those of other species are disposed in 

 fives. The calyx consists of four sepals diverging widely from 

 each other ; and within these, alternating with them, are four 

 petals. The stamens are eight in number ; and they arise from a 



