Sentence:  A basic flower ... has four series of parts arranged in concentric "whorls" (or rings) on the <WSD>"receptacle</WSD>", which is the name given to the expanded end of the "pedicel" (flower stalk). 
Meaning of the word marked with <WSD> tags:  The part of the flower stalk (peduncle or pedicel) to which the floral parts are attached; a thalamus, a torus.