Speed and force of spore ejection in Selaginella martensii

Abstract

Schneller J., Gerber H. and Zuppiger A. 2008. Speed and force of spore ejection in Selaginella martensii. Bot. Helv. 118: 13 – 20.

Spores of the genus Selaginella are discharged through an ejection mechanism caused by the anatomical differentiation of the sporangium. To understand the evolution of these specialised dispersal mechanisms, it is important to know how effective they are, i.e. what distance is reached by the spores, how rapidly they are ejected, and what forces must be developed by the plants to achieve this speed. Here we present a method to determine these important variables. We observed the spore discharge process for Selaginella martensii using a high-speed camera, which allowed us to resolve the movement to 1/1000 s. The mico- and megasporangia opened slowly, separating into two ovoid valves, which subsequently dried and closed in a sudden, quick movement, ejecting the spores. The distances of spore dispersal were determined by placing single shoots of Selaginella on paper in the laboratory. Microspores reached up to 5–6 cm from the spore source, while megaspores reached up to 65 cm from the source, with a mean flight distance of 21.3 cm. Based on the flight trajectories observed on the photographs and the mean weight of a megaspore (1.4 μg), we calculated the speed and forces responsible for the ejection mechanism. The speed of the spores at ejection time was 0.6 m/s for microspores, and 4.5 m/s for megaspores. The initial impulse of one megaspore was estimated as 6.3 pNs. A force greater than 7 μN is necessary to accelerate the megaspore in less than 1 ms to this speed. Our new method to determine the impulse and initial speed of Selaginella spores makes possible more detailed studies about the role and function of biological structures responsible for spore ejection.