2OO Plants and their Ways in South Africa 



name Sympetaly (Gamopetaly, Monopetaly) refers to zonal 

 development of the corolla, and corresponding terms to that of 

 sepals, stamen, and carpels (see p. 141). 



The natural order Compositae, in which cohesion and 

 adhesion is greatest among Dicotyledons, gives evidence to 

 the advantage attending the union of floral parts, if we may 

 judge by numbers. This widely distributed order literally in- 

 herits the earth and is the largest among flowering plants, 

 numbering over 11,000 species. 



Combined carpels economize in pollen. A sympetalous 

 corolla offers a firm support for insects and catches the over- 

 flow from nectaries which it helps to conceal. As development 

 advances the hollow receptacle "fuses" with the ovary which 

 becomes inferior. 



Grouping of flowers to form heads and umbels seems also to 

 further success. It is not only an economy of insect labour 

 but the flowers, sharing the protection afforded by a common 

 involucre, may have the calyx developed as a special mechan- 

 ism in the dispersion of seeds. Each seed in Compositae is 

 provided with its own aeroplane. 



Two-sided symmetry (zygomorphy) is frequent among more 

 highly specialized flowers. The cause of this development is 

 far from clear. In heads and umbels it may be connected 

 with one-sided contact with other flowers. Nutrition, air, and 

 light relations have been suggested as " explanations ". In 

 flowers borne laterally on the stem, it may have been induced 

 by insects' visits. Each flower is zygomorphic before it is 

 visited, so this theory requires us to believe that the effect of 

 the insects' alighting upon it is transmitted through succeeding 

 generations. Gravity and other clauses may have some part in 

 bringing it about. . 



The tendencies shown in the evolution of flowers have been 

 related to economy in pollen, efficiency in pollination, and in 

 seed distribution. Thus far fossil records show Archi- 

 chlamydeae before Sympetalae, and flowers with radial symmetry 

 before zygomorphic ones. 



" The less an organ is concerned with special habits, the 

 more important it is for classification. " Groups of higher rank 



