HOW FLOWERS CLUB TOGETHER. 145 



tral ones, so that they form an external petal-like 

 row, which adds greatly to the attractiveness of 

 the entire flower-head. Of this type, the common 

 blue cornflower is a familiar example. Clearly the 

 plant has here developed the outer florets more 

 than the inner ones in order to make them act as 

 extra special attractions to the insect fertilisers. 



The more familiar type of composites so much 

 cultivated in gardens carries these tactics a step 

 further. We saw reason to believe in a previous 

 chapter that petals were originally stamens, flat- 

 tened and brightly coloured, and told off for the 

 special attractive function. Just in the same way 

 the ray-florets of the daisy, the sunflower, the 

 single dahlia, and the aster are florets which have 

 been flattened and partially or wholly sterilised 

 in order to act as allurements to insects. The 

 ray-floret acts for the compound flower-head as 

 the petal acts for the individual blossom. 



In many other families of plants besides the 

 composites we get foreshadowings, so to speak, of 

 this mode of procedure. The outer flowers of a 

 cluster, be it head or umbel, are often rendered 

 larger so as to increase the effective attractive- 

 ness of the whole ; and sometimes they are sacri- 

 ficed to the inner ones by being made neuter or 

 sterile, that is to say, being deprived of stamens 

 and pistil. Thus in cow-parsnip, which is a mem- 

 ber of the same family as the carrot and the hem- 

 lock, the outer flowers of each umbel are much 

 larger than the central ones, while in the. wild 

 guelder-rose the central flowers alone are fertile, 

 the outer ones being converted into mere ex- 

 panded white corollas with no essential floral 

 organs. But it is the composites that have car- 

 ried this process of division of labour furthest, 

 10 



