REPRODUCTION IN PLANTS 



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rings outside the stamens; (2) the sepals (collectively called the calyx), 

 usually smaller than the petals, often green in color, enclosing the base 

 of the corolla; (3) the receptacle, or expanded tip of the flower stalk to 

 which the pistils, stamens, petals and sepals are attached; and (4) the 

 immediate stalk of the flower, either a peduncle or pedicel, as previously 

 distinguished. This stalk and the receptacle are parts of the axis of the 

 plant. All the remaining parts are modified leaves. In most flowers the 

 pistils stand at the center, and the remaining parts are spirally or con- 



Fig. 17.11. Spiral and cyclic arrangement in flower parts. Left, a mountain buttercup 

 (Trollius albiflorus, family Ranunculaceae). Its petal-like parts are colored sepals and are 

 cyclic in arrangement, but the numerous inconspicuous stamenlike petals, stamens, and 

 pistils are spirally arranged and indefinite in number. Right, a lily (Calochortus species, 

 family Liliaceae), with all parts fixed in number and cyclic in arrangement. (Photos by 

 Prof. Alexander H. Smith.) 



centrically arranged around them — first one or more rows of stamens, then 

 the petals, and on the outside the calyx whorl. 



Modifications in flower structure. Within the limits imposed by the 

 scheme just described, flowers show innumerable variations in structure, 

 arrangement, size, and appearance. They may be large and brightly 

 colored or small and inconspicuous; they may be composed of a large 

 number of parts or of few; they may possess both stamens and pistils 

 or only one or the other of these; petals and sepals may be present or 

 one or both of these sets of accessory structures may be lacking. 



A flower is said to be perfect if it possesses both stamens and pistils; 

 if it has only stamens or only pistils it is imperfect. If it has a full set of 



