LA W OF PROGRESSIVE COLOURATION. 45 



varies from pale pink to mauve : its calyx has become 

 tubular. Yellow blossoms occasionally occur in this 

 genus. But the highest of all' our crucifers are con- 

 tained in the genera Matthiola and Cheiranthus, which 

 have large spreading petals on long erect claws, 

 besides often being sweet scented. The common 

 stock (M. incana) is purple, reddish, or even violet ; 

 our other British species, M. sinuata, is pale lilac ; and 

 no member of the genus is ever yellow. The wall- 

 flower (Cheiranthus ctieirt) is rich orange or red, some- 

 times yellow : its colour, however, differs widely from 

 the primitive golden yellow of the charlocks or 

 buttercups ; and it will receive further attention here- 

 after. 



There is one special (perhaps artificial) tribe of 

 crucifers, the Lomentosa, which display specially high 

 modification in the pod or fruit ; and these deserve 

 separate treatment. Yellow flowers are here very 

 rare ; but one English species, Isatis tinctoria, the 

 dyer's woad, has small yellow petals. Raphanus 

 raphanistrum, the wild radish, has usually in its sea- 

 coast form pale primrose blossoms, much larger than 

 woad ; but inland they are oftener white with coloured 

 veins, and sometimes even lilac. Crambe maritima, 

 the seakale, a somewhat more developed type, is 

 always white, never yellow ; and Cakile maritima, a 

 still higher plant of the same tribe, has purple 

 blossoms, much like those of a stock. 



So much by way of illustration of the families with 

 usually regular polypetalous flowers and free superior 

 ovaries. The other families of this type not noticed 

 here will receive attention in a later chapter. We 

 may next pass on to the families of polypetalous 



