Somatic polymorphism and seed dispersal

Abstract

SOMATIC seed polymorphism is the production of seeds of different morphologies or behaviour on different parts of the same plant and is a somatic differentiation rather than the result of genetic segregation¹. This phenomenon appears to be confined to a limited number of families of higher plants (for example, Cruciferae, Compositae, Chenopodiaceae, Gramineae). Seed produced within a somatic polymorphism may vary in size (Xanthium sp.²), colour (Atriplex heterosperma Bunge³) and/or external structure (Chenopodium album L.⁴). These variations in morphology are frequently accompanied by differences in germination requirements with the consequence that the germination of polymorphic seeds may be staggered in time. Such somatic channelling of progeny into distinct classes of behaviour may increase the fitness of individuals in environments that are temporally patchy, at the expense of the intrinsic rate of increase that could be attained by a unique seed type in a uniform environment. It might be expected that somatic seed polymorphism would also be found in species occupying habitats that are patchy in space and under these circumstances polymorphism in dispersal mechanisms could also be anticipated. One reported case may come in this category; Gymnarrhena micracantha Desf.⁵, a desert annual, produces numerous small aerial wind-dispersed achenes (the fruit in Compositae is usually termed an achene but is strictly a pseudo-nut as the pericarp is partly axial and there is more than one carpel⁶) and also larger subterranean achenes close to the parent plant. The wind dispersed achenes are produced in greater numbers in years with high rainfall. The subterranean achenes serve only to establish descendants in the safe sites previously occupied by their parents. We report here an even more striking polymorphic dispersal mechanism in the biennial Picris echioides L. (Compositae).