Anatomical Structure of the Alfalfa Flower and an Explanation of the Tripping Mechanism

Abstract

1. Eleven alfalfa selections were studied at the blossom stage of development in an attempt to relate proportion of flowers tripped by insect pollinators to certain characteristics of the blossoms. Techniques used to obtain relative measures of resistance to, and force of, tripping of the alfalfa flowers are described. 2. Detailed study of the anatomical structure of alfalfa flowers shows two major forces to be involved in the tripping mechanism. These are (a) the cohesive force between the two keel petals and (b) the pressure exerted by the sexual column from cells under tension at the juncture of the fused filaments and the keel. The former force, resulting from the interlocking of "finger-like" projections on the appressed surfaces of the keel petals, is sufficient to prevent spontaneous tripping of alfalfa flowers. Both the tenacity of this restraining force and the relative pressure exerted by the sexual column against the appressed keel petals differed in flowers from various alfalfa selections. These variations were attributed to differences in anatomical structure and makeup of the blossoms. 3. Evidence indicates that the restraining force of the adhered keel petals is more closely related to the proportion of flowers tripped by insects than is the degree of force with which the sexual column is released on tripping.