84 BRITISH FLOWERING PLANTS chap. 



D. tenuifolia. — Only the two nectaries at the base of 

 the shorter stamens secrete honey. Yet the others are 

 larger. The two sepals outside the secreting glands are 

 upright, the others are spreading. The anthers of the 

 short stamens open inwards. The flowers are visited 

 by flies, bees, and butterflies, but only a few species of 

 each are recorded as visitors. 



D. muralis.— In this species all the glands secrete 

 honey, and all the sepals are similarly oblique. 



Draba 



D. aizoides. — The flowers are at first golden yellow, 

 gradually becoming white. This is the first flower we 

 have come to which changes colour. I will discuss this 

 interesting point later on. When the flower opens, the 

 stigma projects above the anthers, which are shorter 

 and not yet ripe. They gradually elongate and open 

 when they reach the level of the stigma, over which 

 they bend, so that if it has not been visited by insects 

 the flower fertilises itself. Mtiller records in the Alps as 

 visitors 13 flies (7 Muscidse and 6 Syrphidse), 10 butter- 

 flies, and 1 beetle. The plant is not a genuine native ; 

 occurring only on rocks and walls at Pennard Castle, 

 near Swansea. 



Alyssum 



Two species occur in Britain, but neither is a true 

 native — A. calycinum, in which the calyx is persistent 

 and the seeds two in each cell ; and A. maritimum, in 

 which the sepals fall ofi" after flowering, and there is 

 only one seed in each cell. 



In A. calycinum the stamens present difl'erences 

 resembling those which also occur in Allium. The 

 shorter ones have a small appendix at each side. This 

 species is also interesting as being one of those in 

 which the petals change colour, being first yellow and 

 then white. 



The flowers of A. maritimum (Sweet Alison) have a 

 very sweet scent of honey. The seeds are compressed 



