158 MORPHOLOGICAL DEVELOPIVIENT. 



also their outward- growing prolongations — a difference pos- 

 sibly related to some difference in the habits of the insects 

 that fertilize them. Nevertheless, these composite flowers 

 which have inner florets with strap- shaped corollas out- 

 wardly directed, equally conform to the general principle ; 

 both in the radial arrangement of the assemblage of florets, 

 and in the bilateral shape of each floret; which has its 

 parts alike on the two sides of a line passing from the centre 

 of the assemblage to the circumference. Certain 



other members of this order fulfil the law somewhat difier- 

 ently. In Centaurea, for instance, the inner florets are small 

 and vertical in direction, while the outer florets are large and 

 lateral in direction. And here may be remarked, in passing, 

 a clear indication of the effect which great flexibility of the 

 petals has in preventing a flower from losing its original 

 radiate form ; for while in C. cyaniis, the large outward- grow- 

 ing florets, having short, stiff divisions of the corolla, are 

 decidedly bilateral, in C. scahiosa, where the divisions of the 

 corolla are long and flexible, the radial form is scarcely at 

 aU modified. On bearing in mind the probable relations of 

 tlie forms to insect-agency, the meaning of this difference 

 •uill not be difficult to understand. 



§ 236. In extremely- varied ways there are thus re-illus- 

 trated among flowers, the general laws of form which leaves 

 and branches and entire plants disclose to us. Composed as 

 each cluster of flowers is of individuals that are originally 

 similar ; and composed as each flower is of homologous foliar 

 organs ; we see both that the like flowers become unlike and 

 the like parts of each flower become unlike, where the posi- 

 tions involve unlike incidence of forces. The symmetry 

 remains radial where the conditions are equal all round ; 

 shows deviation towards two-sidedness where there is slight 

 two-sidedness of conditions ; becomes decidedly bilateral 

 where the conditions are decidedly bilateral ; and passes into 

 an unsymmetrical form where the relations to the environ- 

 ment are uns^nnmetrical. 



