ANGIOSPERMS 407 



the members of the flower which enforce cross pollination and 

 fertilization. 



Fourth. The structure of the flower. The form of the flower, 

 the number and relation of its members and relation of the flowers 

 in mass, which show an advance from simple and generalized 

 flower types to specialized types and specialized arrangements, 

 form the basis for the grouping of the Angiosperms into classes, 

 subclasses, orders and families. The more primitive flower types 

 probably possessed a large and indefinite number of floral ele- 

 ments, which were arranged in a spiral manner on the floral axis 

 following the arrangement of the leaves, since the floral elements 

 are regarded as modified leaves. Among the lowest of the di- 

 cotyledons the buttercup family (Ranunculaceae) illustrates a 

 simple type of flower structure and arrangement. In some 

 species there are numerous stamens and simple pistils in spirals. 

 The white water lilies show a similar arrangement of the numer- 

 ous members of the perianth and stamens. Among the higher 

 angiosperms there is a tendency to a definite and small number 

 of plant elements. In the monocotyledons this is shown in the 

 trimerous flowers (parts in three's) of so many families, and in 

 the dicotyledons by the tetramerous (parts in four's) and pentam- 

 erous (parts in five's) flowers, and by the floral elements being 

 so crowded on the short axis that they no longer are in spirals but 

 in whorls or cycles (cyclic arrangement). This specialized con- 

 dition, shown in the reduction from numerous parts, to a few 

 definite in number, is regarded as showing a higher state of evo- 

 lution. It is often accompanied by the loss of certain parts in 

 some flowers, which leads to two different kinds of flowers, on 

 the same plant or on different plants, shown in the ray-flowered 

 composites; also in the difference in length of the same essential 

 organs in different flowers (dichogamy) or in a difference in time 

 of maturity (proterandry, proterogyny). In the more primitive 

 flowers the parts are all separate, and the flowers are actinomor- 

 phic or radiate in the arrangement of the parts (radial symmetry). 

 As the flower has become more specialized, a change in form to 

 bilateral symmetry has taken place in many groups, as in the 



