LA W OF PROGRESSIVE COLOUR A TION. 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 cheirt) 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 LomentoscB^ 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 tinctorial 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 



