2 • Background 



borne by many true angiosperms, which are commonly called "male 

 flowers"— that is, structures containing only the pollen-bearing 

 stamens and without even rudimentary carpels. In practice, the 

 restriction to carpel-bearing structures need not apply here. Studies 

 of flowering in gymnosperms such as pines have been conducted 

 and are, for physiological purposes at any rate, analogous to studies 

 on angiosperms. For these purposes, then, flowering can be taken 

 to mean the production of sporogenous shoots by either angio- 

 sperms or gymnosperms; the term flower in its common usage will 

 not be misleading. 



The parts of "typical" flowers— such as those found in botany 

 texts— are usually described as follows: the floral axis is more or 

 less shortened as compared with that of a vegetative shoot, and 

 bears successive whorls of parts arranged around it. The structure 

 on which the flower parts are placed is the receptacle, and the 

 stalk bearing the flower is the pedicel. The lowest or outermost 

 parts are the sepals, commonly enclosing the bud; within and above 

 are the petals. Sepals and petals are collectively the perianth. 

 Within this are the stamens, each consisting of a filament bearing 

 a pollen-producing anther. The upper or innermost flower parts 

 are the carpels, which, either singly or united, give rise to one or 

 more ovaries, containing the ovules, and to a pollen-receptive sur- 

 face, the stigma. Stigma and ovary together, whether derived from 

 one or more carpels, are called the pistil. 



Many individual flowers often occur on a single simple or 

 complex axis as in the sunflower (Helianthus) or in grasses; such a 

 group of flowers is an inflorescence. Flowers may also be solitary, 

 each borne on a separate pedicel attached to the vegetative axis. 

 Flowers or inflorescences may be terminal (at the ends of shoots) or 

 lateral, or both, and may also be enclosed or accompanied by leafy 

 or scaly bracts. 



There are great differences between various plants in the num- 

 ber, arrangement, shape, size, color, degree of fusion, and even 

 presence or absence of the various flower parts. In spite of this, 

 there is a good area of agreement among botanists both on the 

 phylogeny, or evolutionary origin of the flower, and on its ontogeny, 

 or development from the vegetative axis in the individual plant. 



The definition of a flower as a particular kind of determinate 

 shoot already implies an interpretation of both phylogeny and 



