86 RHAMNACE&, LEGUMINOS^E. [CHAP. 



(the Buckthorn) with two species, Rhamnus cathar 

 ticus (the common Buckthorn and Rhamnus frangula), 

 the Alder Buckthorn. The two species differ con- 

 siderably. In Rhamnus frangula, the sepals, petals 

 and stamens are five in number ; the petals are 

 very small. The stamens open before the stigma 

 is fully developed, and probably even before it is 

 capable of fertilisation. The pistil is in the centre, 

 and insects which visit the flower for the sake of 

 the honey necessarily touch the stamens with one 

 side of the proboscis and the pistil with the other. 

 They must, therefore, often convey the pollen from 

 one flower to another. In the absence of insects, 

 however, R. frangula is capable of self-fertilisation. 



In R. catharticus, on the contrary, the flowers have 

 four petals, and are dioecious ; the male flowers have a 

 rudimentary pistil, and the female flowers bear minute 

 stamens. The individual flowers are very small, they 

 are rendered conspicuous by being associated in 

 clusters, while those of R. frangula are in twos or 

 threes. 



R. lanceolatus, which, however, is not an English 

 species, has been shown by Mr. Darwin to be dioecious 

 (Jour. Linn. Soc., v. vi., 1862, p. 95.) 



LEGUMINOS^E. 



This is a very extensive order, containing eighteen 

 British genera ; the Peas, Vetches, Brooms, Clovers, 

 Furze, &c. belong to it. 



It is probable that all flowers which have an 



