THE EVOLUTION OF THE FLOWER 393 



of the pollen grains are simply carried one step farther, and 

 the megasporophylls (carpels) become factors in the processes. 

 Thus a receptive surface, the stigma, was developed as a special 

 organ to receive and start the pollen tube in its parasitic devel- 

 opment, which is to end with the fertilization of the egg. 



The seed is the ripened ovule, for the principle of protection 

 is continued after fertilization, and the integuments form hard 

 seed coats, inclosing the developing embryo, supplied with food 

 material by the parent sporophyte until it has reached an ad- 

 vanced stage of development. 



THE EVOLUTION OF THE FLOWER 



368. The evolution of the flower.* The higher types of 

 flowers have been developed by long processes of evolution from 

 the simpler structure of the primitive flowers. We do not 

 know exactly what the primitive flowers were like, but some 

 of their characters may be inferred from the structure of the 

 simplest flowers of the angiosperms and the cones of gymno- 

 sperms and certain pteridophytes, as the horsetails and club 

 mosses, which are truly flowers, if one accepts the definition of 

 a flower as a " shoot beset with sporophylls." The most elabo- 

 rately developed theory of floral evolution is that of Engler, and 

 this brief outline will be a general statement of his views. 



Primitive flowers were characterized by indefinite numbers 

 of sporophylls, usually distributed in spirals, and the absence of 

 the floral envelopes constituting a perianth. These conditions 

 are illustrated in the cones of the pteridophytes and in many 



* To THE INSTRUCTOR : This subject should only be presented to classes 

 with a fairly wide range of experience with flower structure in various 

 groups of angiospern.s. An excellent study would be a series of types from 

 such an assemblage as the buttercup order, Ranunculales, as the mouse- 

 tail (Myosurus), buttercups, magnolia, white water lilies, columbine, lark- 

 spur, aconite, etc., where many of the principles of flower evolution are 

 illustrated in a single group. Similar studies might be planned for the rose 

 order, Rosales, or the lily order, Liliales, followed by the orchids. 



