374 PLANT MORPHOLOGY 



set, which may or may not be true. In fact, the single whorl apparently 

 often represents a perianth that has never become differentiated into a 

 calyx and corolla. Naked flowers are those which are entirely without a 

 perianth. It may have been lost through degeneration or may never have 

 been developed. 



Sepals and petals are leaf-like in both form and structure. Phylogenet- 

 ically they may either have been derived by sterilization from sporophylls. 



Fig. 319. Flower of Magnolia yrandijlora, a primitive type, with numerous stamens and 

 carpels borne in spiral arrangement on an elongated receptacle, one-lialf natural size. 



may represent modified foliage leaves, or possibl}^ at least in some flowers, 

 the sepals may have evolved from leaves and the petals from stamens. 

 Often the foliage leaves and perianth parts intergrade, making it difficult 

 to delimit the flower from the vegetative shoot that bears it. In some 

 flowers, notably in the water lily (Nymphaea), the petals intergrade with 

 the stamens. 



Most flowers have a regular (actinomorpMc) corolla, composed of petals 

 alike in size and shape, the flower as a whole exhibiting radial symmetry 

 (Fig. 318). This represents a relatively primitive condition. Many 

 flowers have an irregular {zijgomorphic) corolla, with not all the petals 

 alike, thus showing bilateral symmetry. This tendency reaches its high- 

 est expression in flowers having spurs, sacs, or pouches, as in the Legu- 

 minosae, Labiatae, and Orchidaceae. 



Establishment of Whorls and Definite Numbers. Primitive flowers, 

 like those of the magnolia and buttercup (Ranunculus), have a convex, 

 elongated receptacle bearing indefinitely numerous stamens and carpels 



