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JUNE 2006 



Origins 



of Floral Diversity 



A quarter-million flowering plants attest to a highly flexible 



developmental recipe. Plant biologists have now proposed 



a genetic model that may account for the profusion of floral forms. 



By Amy L/'tt 



Colorful petals, sweet perfumes, and delicate 

 shapes make flowers a delight to the senses. 

 Each of the 250,000 species of flowering 

 plants — the plant division known as angiosperms — 

 makes a distinct flower, and the resultant diversity 

 boggles the mind. Showy, exuberant flowers — roses, 

 lilies, orchids — catch peoples attention, but many 

 plants flower without making such a fuss of it: think 

 of the oaks, maples, and grasses. Flowers may grow 

 singly, as tulips do, or with companions on a com- 

 mon stem, called an inflorescence, as the banksia 

 does [see rightmost photograph on opposite page]. They 

 may be radially or bilaterally symmetrical, tubular in 

 form or dish-shaped. Flowers may lack one or an- 



other kind of organ altogether (poinsettias, for in- 

 stance, lack petals — their showy display is formed 

 out of modified leaves that eclipse small, petalless 

 flowers), or yield a profusion of them. Each kind of 

 organ, moreover, may be fused or separate, standard 

 issue or tricked out to entice a specific pollinator. 

 Petals may be fringed, spiky, or spurred; stamens 

 may be stiff, droopy, or jiggly; the variations go on 

 and on. Most people are content to take pleasure in 

 the sheer abundance and variety of flowers, but we 

 evolutionary botanists are less easily gratified. What 

 is the origin of such extraordinary diversity of 

 flower form, we wonder? What is the genetic basis 

 of all this evolutionary innovation? 



Flowers exhibit lavish variety in shape and color, as well as in the number and 

 arrangement of their organs. The flowers pictured here, not all shown to the 

 same scale, suggest some of that diversity. They are (left to right): passionflower, 

 tulip, Queen Anne's lace (in background, throughout), poppy, tulip, thistle, 

 chrysanthemum, and banksia. What may appear to be a single flower in Queen 



