CH. XI] COROLLA AND CALYX. 145 



which gives a floral diagram, or simplified bird's-eye view 

 of a flower 1 ; the two outer whorls are represented by 

 brackets, the sepals being shaded, while the petals are 

 black. In spite of the overlapping of the parts it is clear 

 that the centre of any petal is half-way between the 

 centres of two sepals. The importance of the alternation 

 of petals and sepals will appear when the structure of the 

 bean-flower is examined. 



In the buttercup the petals are bright yellow, while 

 the sepals are less bright in tint ; in the bean the petals 

 are black and white, while the calyx is nearly colourless. 

 This is a general but by no means an absolute rule, 

 namely that the calyx is green or dingy in colour, 

 while the petals are conspicuous. It is also commonly 

 the case that the sepals are of a simpler, less elaborate 

 pattern than the petals: this is not obvious in the 

 buttercup, where both are of a simple form; but in 

 the bean the contrast is plain, the petals being of 

 a specialised type, while the sepals are simple. A com- 

 parison of these flowers brings out another important 

 point, namely that each sepal may be free, i.e. not 

 united to the neighbouring sepals, as in the buttercup ; 

 or the sepals may be united into a cup or tube, as 

 in the pea (fig. 67). Similar differences in regard to 

 cohesion occur in the petals : thus in the cowslip they are 

 united into a tube (fig. 65), while in the buttercup they are 

 free. Other opportunities will occur of considering this 

 point, which is here merely noted as one of the most 



1 The floral diagram, though not that of a Eauunculus, serves equally 

 well to illustrate alternation. 



D. E. B. 10 



