206 APPLIED BIOLOGY 



flowers which are not radially symmetrical, as are those of 

 apple, buttercup, tulip, and many others equally well known. 

 In the irregular flowers of bean, sweet pea, violets, pansy, 

 salvia and many others the petals are separate; but there 

 are many irregular flowers with united petals. Flowers of 

 toad-flax, snapdragon, mullein, catalpa, and any mint are 

 good examples. In most cases irregular flowers appear to 

 be adapted to insect visits. This does not mean that regular 

 flowers became irregular because insects began to visit them, 

 nor in order to allow or to encourage insects to make visits ; 

 but rather that flowers which were irregular were thereby 

 fitted to insect visits. Just what caused flowers to become 

 irregular in the first place is unknown. 



201. Double Flowers. A large number of varieties of 

 cultivated flowers have become " double," which means 

 that the petals have become very numerous and the stamens 

 and pistils have largely or entirely disappeared. In some 

 varieties (e.g., the double-flowered portulacas) some flowers 

 do not develop completely " double," but have pistils and 

 stamens which produce seeds. When these seeds develop 

 into new plants, some of the flowers produced will be 

 " double " and others only partly so. It is impossible to 

 continue to raise only completely double flowers from seed, 

 for they would be seedless. If a gardener wants a bed of such 

 flowers, he must propagate entirely from cuttings. In the 

 case of many varieties of roses this propagation from cuttings 

 is the exclusive method used by gardeners, and we never 

 see stamens and pistils in flowers of these varieties. 



The explanation of double flowers is often said to be that 

 the stamens and pistils have changed to petals. One fact 

 which at first sight seems to support this view is that in 

 some flowers (e.g., water-lily) there are all stages of transition 

 between petals and stamens. However, this does not prove 

 that stamens and pistils were originally petals. On the con- 

 trary, stamens and pistils appeared in the lowest flowers 



