مشخصات پژوهش

tWe studied the inflorescence, and in particular ontogeny and development of the florets in Senecio vernalisas a representative member of Asteraceae, using epi-illumination microscopy. Initiation and subsequentdevelopment of florets on the highly convex inflorescence apex occur acropetally, except for pistillateray florets, which show a lag in initiation. Receptacular bracts derive from the receptacular surface afterdevelopment of all florets. The order of whorl initiation in both disc and ray florets include corolla,androecium and finally the pappus, together with the gynoecium. Development of corolla lobes froma ring meristem occurs in bidirectional order starting from the lateral side, whereas stamens inceptunidirectionally from the abaxial side. Concurrently with the inception of two median carpel primordia,a ring meristem develops at the base of the corolla from which pappus bristles differentiate in later stages.Pistillate ray florets show significant differences from perfect disc florets as reflected by the zygomorphicshape of the floral apex and a shift of floral merosity from pentamery to tetramery. Loss of stamens inray florets occurs due to abortion of primordia after initiation.