174 BRITISH FLOWERING PLANTS chap. 



cleistogamic flowers, Darwin was disposed' to regard 

 tliem as indicating the manner in which cleistogamy 

 may have originated. 



EOSACEiE 



Of this great family we have in Britain seventeen 

 genera. Both calyx and corolla consist of separate 

 leaves, and the flowers are open, but in other respects 

 present great difi"erences. They are generally large, 

 but sometimes quite small and greenish ; often with 

 honey, secreted by a ring round the base of the calyx, 

 but sometimes without ; sometimes single, sometimes in 

 pairs, in close heads, in cymes, panicles, racemes, 

 corymbs, or spikes ; mostly fertilised by insects, but 

 in some cases wind-flowers ; some are protandrous, some 

 protogynous ; some are considerable trees, some small 

 herbs. In many species the flowers, protect the pollen 

 by bending over in rain. The fruits present many 

 differences ; in some species a pome, in others a drupe, 

 a berry, or an achene. In many of the Rosacese 

 belonging to various genera (Crataegus, Geum, Pyrus, 

 Potentilla, etc.) the stipules are polymorphic. 



Prunus 



Of this genus we have four species : P. spinosa, 

 with the flowers single or in pairs ; P. Cerasus and 

 P. Avium, with flowers in clusters, and P. Padus, in 

 axillary racemes. According to H. Miiller the flowers 

 are homogamous, but Kirchner says they are pro- 

 tandrous. Each flower lasts seven to eight days. P. 

 spinosa and P. Padus are protogynous. In P. spinosa 

 the young leaves are protected by woody spines. 



P. Cerasus (Cherry). — In the wild cherry the fruit is 

 black. It seems probable that it is an advantage for 



' Forms of Flowers. 



