豚草花序及其演化

The Inflorescence of Ragweeds (Ambrosia L.) and its Evolution

The inflorescence of ragweeds (Ambrosia) is a highly specialized type within Compositae. Their male and female florets composed indiv- idual staminate and pistillate heads (capitula) respectively. Staminate heads are arranged in acropetally maturing,ameatlike raceme at the tips of the stems and branches. Pistillate heads occur in basipetally (centr- ifugally) maturing clusters in the axils of leaves immediately below the staminate spikes. The inflorescence of ragweeds underwent five evolut- ionary stages within the Compositae: (1) primitive stage; (2) producing primare capitulous braches; (3) the order of maturation of the primary capitulous braches changing;(4) dividing primary braches into two types; (5) perfect stage. The evolutionary tendency of the heads and floral structures appears to be as follows: (1) florets from bisexual to uni- sexual; (2) heads from possesing both male and female florets to only possesing one of them; (3) involucre from separate to united bracts; (4) staminate heads from upright to downward; (5) spine arrangement on the pistillate heads from scattered in several whorls to localized in a single, apica whorl; (6) number of female florets in pistillat heads from several to one. Some modifications of the inflorescence and floral struc- tures of the ragweeds are obviously associated with adaptation for anemophily.