Seed mass is the mass of the embryo, endosperm and seed coat (Usher 1966). However, some ecologists measure diaspore mass or reserve mass instead of the mass of true seeds (see below).
Seed mass is a crucial component of a species’ regeneration strategy. Seed mass is inversely related to the number of seeds a plant can make for a given amount of canopy in a year (Aarssen and Jordan 2001; Henery and Westoby 2001), and is correlated with seedling survival through many of the hazards commonly encountered during establishment (Leishman et al. 2000), the probability of species having a persistent seed bank (Thompson et al. 1993), and seed dispersal syndrome (Moles et al. 2005a). Seed mass is also correlated with many central life history traits including plant size, time to first reproduction and lifespan (Moles et al. 2004).
Terminology and equations
A “Seed” consists of the embryo, endosperm and seed coat.
A “Diaspore” is the dispersed unit, and includes structures such as wings, pappus hairs and elaiosomes in addition to the true “seed”. However, diaspores are often referred to as “seeds” in the literature, especially in cases such as the achenes of Asteraceae, where the maternal tissue fused to the seed coat cannot be readily distinguished from the true seed. Diaspore mass is often used in studies of dispersal and seed predation, as the diaspore is the unit that must be transported or handled by dispersal agents and predators.
The “Reserve mass” is the mass of the embryo and endosperm, and is a measure of the amount of resources that will be available to the seedling during establishment.
Seed mass is quantified simply by weighing seeds on a suitably accurate balance. If seeds are very small, or an accurate balance is not available, seeds can be weighed in batches (e.g. to determine 100 seed weight). Seeds are weighed at maturity, and unfilled or predated seeds are excluded from the sample. To determine a species’ seed mass, a minimum of five seeds are taken from each of a minimum of three individual plants (Cornelissen et al. 2003).
Some ecologists measure fresh mass, while others prefer to measure air dry or oven dry mass. The most appropriate measure depends on the question. For questions about maternal investment or seedling provisioning, dry mass is the most appropriate measure. Fresh mass may be more appropriate for questions about dispersal or seed predation.
Seeds are oven-dried to constant mass, or left in a moderately hot (usually 50 to 80 degrees Celcius) oven for 1-5 days. Once seeds are removed from the oven, they should be cooled in a desiccator, to reduce the amount of water absorbed from the air (Cornelissen et al. 2003).
Seed mass has been estimated from seed volume in some studies (especially paleontological studies). The precise equations used vary slightly, but 1mg is approximately equal to 1mm3 (Moles et al 2005b).
Ranges of values
The smallest seeds on earth are made by orchids and some hemiparasitic plants, whose seeds weigh just 0.0001mg. These seeds are like tiny grains of dust, and do not contain enough reserve tissue to establish a seedling without help from symbionts or host plants. The largest seeds on earth are produced by the double coconut (Lodoicea maldivica), and may weigh up to 20kg. Across all of the species on earth, there are 11.5 orders of magnitude of variation in seed mass (Moles et al. 2005b). There is huge variation in seed mass among coexisting species too – most ecosystems have species spanning at least 5 orders of magnitude of variation in seed mass (Leishman et al. 2000). Variation at the within species level is much smaller, with most species having 2-fold to 4-fold variation in seed mass. Most of this within-species variation lies at the level of the maternal plant rather than between plants or populations (Leishman et al. 2000).
Health, safety and hazardous waste disposal considerations
Many species’ seeds contain toxic defense compounds.
Aarssen, L.W. & Jordan, C.Y. (2001) Between-species patterns of covariation in plant size, seed size and fecundity in monocarpic herbs. Ecoscience 8, 471-477.
Cornelissen, J.H.C., Lavorel, S., Garnier, E., Diaz, S., Buchmann, N., Gurvich, D.E., Reich, P.B., Ter Steege, H., Morgan, H.D., Van Der Heijden, M.G.A., Pausas, J.G. & Poorter, H. (2003) A handbook of protocols for standardised and easy measurement of plant functional traits worldwide. Australian Journal of Botany 51, 335-380.
Henery, M. & Westoby, M. (2001) Seed mass and seed nutrient content as predictors of seed output variation between species. Oikos 92, 479-490.
Leishman, M.R., Wright, I.J., Moles, A.T. & Westoby, M. (2000) The evolutionary ecology of seed size. Seeds – the Ecology of Regeneration in Plant Communities (ed M. Fenner), pp. 31-57. CAB International, Wallingford.
Moles, A.T., Falster, D.S., Leishman, M.R. & Westoby, M. (2004) Small-seeded species produce more seeds per square metre of canopy per year, but not per individual per lifetime. Journal of Ecology 92, 384-396.
Moles, A.T., Ackerly, D.D., Webb, C.O., Tweddle, J.C., Dickie, J.B., Pitman, A.J. & Westoby, M. (2005a) Factors that shape seed mass evolution. Proceedings of the National Academy of Sciences 102, 10540-10544.
Moles, A.T., Ackerly, D.D., Webb, C.O., Tweddle, J.C., Dickie, J.B. & Westoby, M. (2005b) A brief history of seed size. Science 307, 576-580.
Thompson, K., Band, S.R. & Hodgson, J.G. (1993) Seed size and shape predict persistence in the soil. Functional Ecology 7, 236-241.
Usher, G. (1966) A Dictionary of Botany. Constable & Co Ltd, London.