豚草花序及其演化

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

豚草属(Ambrosia L.)植物花序是菊科植物中高度特化的类型。它们的雄花和雌花分别组成雄头状花序和雌头状花序。雄头状花序排列在枝梢呈葇荑花序状,向顶成熟。雌头状花序发生在紧靠雄穗基部叶腋内,向基(离心)成熟。豚草花序的形成在菊科内部经历了五个演化阶段;(1)原始阶段;(2)一级花序分枝出现阶段;(3)一级分枝发育方向转向阶段;(4)一级分枝分化阶段;(5)演化完成阶段。头状花序和小花结构演化趋势表现为;(1)花由两性变单性;(2)头状花序由雌雄同序到异序;(3)总苞由分离到联合为一体;(4)雄头状花序由朝上到朝下;(5)雌花序总苞刺由多轮散生到集中为顶端一轮;(6)雌头状花序由雌花数枚到一枚。豚草花序和小花的某些变化明显地与风媒传粉有关。

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.