Chap. V. CORRELATION OF GROWTH. 145 



difference in the ray and central florets of, for instance, 

 the daisy, and this difference is often accompanied with 

 the abortion of parts of the flower. But, in some Com- 

 positous plants, the seeds also differ in shape and sculp- 

 ture ; and even the ovary itself, "with its accessory parts, 

 differs, as has been described by Cassini. These differ- 

 ences have been attributed 1 7 some authors to pressure, 

 and the shape of the seeds in the ray-florets in some 

 Compositae countenances this idea ; but, in the case of 

 the corolla of the Umbelliferae, it is by no means, as Dr. 

 Hooker informs me, in species with the densest heads 

 that the inner and outer flowers most frequently differ. 

 It might have been thought that the development of 

 the ray-petals by drawing nourishment from certain 

 other parts of the flower had caused their abortion ; but 

 in some Compositae there is a difference in the seeds of 

 the outer and inner florets without any difference in the 

 corolla. Possibly, these several differences may be con- 

 nected with some difference in the flow of nutriment 

 towards the central and external flowers: we know, 

 at least, that in irregular flowers, those nearest to 

 the axis are oftenest subject to peloria, and become 

 regular. I may add, as an instance of this, and of a 

 striking case of correlation, that I have recently observed 

 in some garden pelargoniums, that the central flower of 

 the truss often loses the patches of darker colour in the 

 two upper petals ; and that when this occurs, the adhe- 

 rent nectary is quite aborted ; when the colour is absent 

 from only one of the two upper petals, the nectary is 

 only much shortened. 



With respect to the difference in the corolla of the 

 central and exterior flowers of a head or umbel, I do 

 not feel at all sure that C. C. Sprengel's idea that the 

 ray-florets serve to attract insects, whose agency is 

 highly advantageous in the fertilisation of plants of 



H 



