THE DICOTYLEDONES 



1957 



the flowers are mainly visited by insects belonging to what are generally 

 regarded as the more advanced orders of insects. If it is correct to suppose 

 that insects and flowers were evolved simultaneously side by side, that 

 state of affairs is what we might expect. 



Apart from particular pollination mechanisms to which we shall refer 

 later, the success of the Compositae lies generally in the aggregation of the 

 flowers into compact heads whereby cross-pollination is almost inevitable. 

 At the same time the inflorescences are conspicuous, which is accentuated 

 in various ways, either by the florets being outwardly directed, e.g., in 

 Centaiirea, or by the ligulate character of the corolla becoming exaggerated 

 towards the margin of the capitulum, or in other cases by the development 

 of distinct ray florets or by the presence of an inner series of conspicuous 

 involucral bracts as in Carlina. 



Further evidence of the success of the family is seen in the efficient fruit- 

 dispersal mechanism occurring in most species. The pappus itself is a 

 highly efficient organ, which is sometimes further improved by the presence 

 of hooks. Distribution by water is also made possible by the imprisonment 



Fig. 1886. — Siegesbeckia orientalis. Flowering shoots. 



of air between the hairs of the pappus and by its unwettable nature, thus 

 producing a floating mechanism. Hooked fruits of various kinds also occur, 

 while in a few, sticky glandular hairs are produced on the fruits, as in 

 Siegesbeckia orientalis (Fig. 1886), a plant widely distributed in tropical 

 regions the world over. Certain fruits are polymorphic in the same capi- 

 tulum as is seen in Calendula officinalis (Marigold) where the outermost 

 fruits derived from the outer female flowers are much elongated with 

 small spines on the back, while fruits nearer the centre have broad wings 

 inrolled at the margins and fruits at the centre of the capitulum are incurved, 

 often forming a ring, and have narrow involute wings. 



The economic importance of the family is considerable and we have 

 already referred to a number of examples. Many contain valuable fatty 



