476 CONVOLVULUS TRIBE. 



liancy, that the eye can scarcely rest upon them. Several genera 

 of this order are common in Britain ; the chief are the Gentian, 

 and the Centaury. The whole order is characterised by the 

 presence of a very bitter principle, which exists in the leaves, 

 flowers, bark, and roots ; this is much valued in medicine as a 

 tonic. 



65 1 . Another well-known order, chiefly important on account 

 of the valuable medicinal products which it yields, is that of 

 CONVOLVULACE^J, the Convolvulus or Bindweed tribe. The species 

 with which we are familiar in this country, are for the most part 

 trailing plants, depending for support upon others ; but between 

 the tropics, where the order especially abounds, we find it con- 

 taining also standard shrubs and even trees. The calyx and 

 corolla in this order each consist of five divisions; the sepals 

 usually overlie each other so completely, however, that only the 

 two outer ones can be distinguished. Very commonly the petals 

 are so completely adherent, that they do not separate even at 

 their extremities ; this we notice in the common Bindweed of 

 our hedges. This order is remarkable for the curious plaiting 

 into which the corolla folds when it closes. In most species it 

 opens and closes under the influence either of light or of dark- 

 ness, some opening only in the day, and others only at night ; 

 and there is one curious species, in which the flowers are so sen- 

 sitive as to contract beneath the touch, like the leaves of the 

 Sensitive plant. The stamens are five in number, and are oppo- 

 site to the sepals of the calyx. The ovarium usually contains 

 three or four cells, in each of which there are but one or two 

 ovules ; the style is single, but divides into two stigmas. The 

 fruit is a capsule, opening by valves equal in number to the divi- 

 sions of the ovarium. The form of the embryo is curious, and 

 serves to distinguish this order from others that are nearly allied 

 to it in the preceding characters; it is doubled up in a remark- 

 able manner, as if there were not room within the seed for it to 

 grow ; and the cotyledons are small and shrivelled, being occa- 

 sionally altogether absent. The greater number of species may 

 also be distinguished from other monopetalous plants, by their 

 milky juices ; but this character is less apparent in the annual 



