Evolutionary Trends in the Inflorescence of Angiosperms

Abstract

The most primitive type of inflorescence in angiosperms is believed to be a leafy cyme. Examples from Magnoliaceae, Winteraceae, Annonaceae, Dilleniaceae, Ranunculaceae, Papaveraceae and Rosaceae indicate that even in these relatively primitive families the solitary flower is a derived condition. Determinate inflorescences are regarded as more primitive than indeterminate ones, a conclusion that is supported by examples from the Ranunculaceae, Papaverales, Rosaceae, Saxifragaceae, Apocynaceae, Campanulales and Violaceae. Trends of reduction and of amplification are both illustrated by examples. In the Compositae and Gramineae, two completely different patterns of development are superimposed upon each other, resulting in complex types of compound inflorescences. The ontogenetic development of inflorescences is related to the timing of the transformation of the shoot apex from the histological and physiological properties characteristic of its vegetative condition to the very different properties characteristic of the reproductive shoot apex. If this transformation coincides with or is immediately followed by the differentiation of the primordial floral appendages, a solitary flower or a simple leafy cyme is produced. In plants having very complex inflorescences, such as the capitulum of the Compositae and the panicle of the Gramineae, the transition occurs before the differentiation of separate floral primordia. In addition, reproductive apices that form complex inflorescences, containing bracts that are strongly differentiated from foliage leaves, are more strongly differentiated from vegetative apices than are reproductive apices that produce relatively simple, less well differentiated inflorescences. The distinctness of the inflorescence relative to the vegetative shoot depends to a large extent upon the abruptness of the transition of the shoot apex from the vegetative to the reproductive condition. The shape of the inflorescence is correlated with the orientation of mitoses in the primordial reproductive apex. The origin of indeterminate from determinate inflorescences has often been a two step process: reduction followed by amplification. These steps have been guided by alternating selective pressures: first for reduction of the length of the reproductive cycle in response to a shorter growing season; and second for increase in seed production in response to the selective pressures of a more favorable environment. Elongate racemes are most adaptive when the reproductive cycle is relatively long, but conditions of growth are less than optimum, because of low light, shortage of minerals, or other restrictions. Inflorescences that have repeated sympodial branching are highly flexible in their reaction to the environment, and so are particularly well adapted to regions where the favorable growing season fluctuates greatly from one year to another. The evolution of inflorescences illustrates very well two general properties of evolutionary trends: conservation of organization and adaptive modification along the lines of least resistance.