The giant lobelias: toxicity, inflorescence and tree-building in the Campanulaceae

An hypothesis on the origin of (i) the distribution of alkaloids and thorns, (ii) the position of the inflorescence, and (iii) arborescent forms in the Campanulaceae, considered in the light of recent studies on Lobelia sect. Rhynchopetalum (Fres.) Benth. & Hook, f., is proposed. It is suggested that the primitive Campanulaceae were pachycaul forest plants and that it is possible to argue that on the continents those with alkaloids (Lobelioideae) were tolerant of browsing herbivores until the rise of alkaloid tolerance in herbaceous mammals when only armed forms could survive in lowland rainforest. In the Lobelioideae., the terminal inflorescence is considered more primitive than the lateral which can be derived from the lowermost branches of the terminal with the onset of periodic inception of fertile primordia. In this way, non‐hapaxanthic shoots and the capacity for ‘tree‐building’ arise.