xii INTRODUCTION 



(3) elevation of corolla or stamens or of both, (4) change of form and 

 arrangement, producing irregularity. The sepals may be united to each other 

 into a cup-shaped calyx, as in the Pinks or Gentians (Plate 18), and this 

 may in turn be united or grown to the ovary, as in the Evening Primroses 

 and Mentzelias (Plate 32). They may become irregular in shape and 

 brightly colored, as in the Buttercups (Plate 1), or they may be reduced 

 in number. The petals may become united to each other, either partly or 

 entirely, as in the Morning Glories and Phloxes (Plate 19). They may 

 grow up and on the ovary as in the Bluebells and Parsleys (Plate 36). 

 They may become either one or all irregular in shape, as in the Irises and 

 Orchids (Plate 43), or the Snapdragons (Plate 22), the Pentstemons (Plate 

 23), and the Mints (Plate 24), or they may be reduced in number or even 

 lost entirely, as in the Willows (Plate 12). So also the stamens may grow 

 together, in whole or in part, the union taking place either by the filaments 

 or the pollen-sacks. They may grow together with the petals, thus becom- 

 ing attached to them. They may be reduced in number and even be lacking 

 in the pistillate flowers of a monoecious or dioecious species. Finally the 

 pistils may become united into a several to many-celled compound pistil, 

 or the cross-walls may disappear between the cells, leaving a 1 -celled com- 

 pound pistil. The number of simple, separate pistils may vary from many 

 to one, but there is never more than one compound pistil in any one flower. 

 Of the utmost importance is the union of the simple pistils and the num- 

 ber of cells in the compounded pistil. It will be seen by reference to the 

 chart, that this union is the first step of advance above the Buttercups. 



The setting apart of flowering plants in orders and families is based 

 upon the kind and degree of change apparent in the structure of the 

 flowers. Moreover, a study of flowers in the field will convince one that 

 the above changes have been brought about as a result of the visits of 

 bees, butterflies and other insects in search of nectar and pollen. Since 

 insects are attracted to flowers largely by color, the corolla is the part 

 most directly affected. For example, uniting the separate petals into a 

 continuous corolla makes a broader and hence more attractive expanse of 

 colored surface. Raising the corolla, which occurs when the petals grow 

 from the top of the ovary, makes it more conspicuous, as does also any ir- 

 regularity in shape. It will be seen from the chart that three of the four 

 steps of advance, which distinguish the higher from the lower groups, affect 

 the corolla. It may also be noted by referring to the flower formulas that 

 these changes in the corolla are almost invariably accompanied by changes 

 in the stamens and sepals. When the corolla is united, as shown by the 

 parenthesis about the number of petals, it will be seen that the calyx also 

 shows this union, and that the stamens are attached to the corolla tube, 



