Seeds

Seed size

The size of seeds interests comparative ecologists because it is so variable among species. The dry mass of seeds ranges over at least six orders of magnitude across species of tropical rain-forest tree. The Melastomataceae and Rubiaceae include tropical tree species with seeds of dry mass as little as 20 µg (Metcalfe & Grubb 1995; Grubb & Metcalfe 1996). At the other extreme, the seeds of a number of trees, notably legumes, approach 100g dry mass. Within any tropical forest site, most studies have shown ranges of least five orders of magnitude for tree seed mass (Hammond & Brown 1995; Metcalfe & Grubb 1995; Grubb & Coomes 1997; Lord et al. 1997). Of course, mass is a volume-dependent property and so will rise with the cube of the linear dimensions involved, which will rapidly exaggerate size differences between species, but a million-fold range in offspring size is still enormous when compared with animal groups.

Seed size might be under allometric control of other characters. There is evidence of correlations with other size variables. The difficulties of small plants producing big seeds and of small fruits containing big seeds will probably always lead to some degree of positive correlation between plant size and seed size (Fig. 5.1). For tropical trees, a number of studies have shown increases in seed size with adult stature within a particular forest (Hilty 1980; Foster & Janson 1985; Metcalfe & Grubb 1995; Hammond & Brown 1995; Kelly 1995; Grubb & Coomes 1997).