The two main proximate causes of intratree variation in individual seed mass of lodgepole pine [Pinus contorta var. latifolia (Dougl. ex Loudon)] were space constraints within cones and between-branch position effects. Cone scales get smaller near the distal tip of the cone. Seed predation has selected for the production of the fertile zone in the distal tip of the cone where cone scales are smaller. Thus, seed and wing size were constrained by cone scale size, and decreased from the base to the tip of the cone. Heavier seeds had larger seed wings, but wing loading (fruit mass/wing area) was independent of seed mass. Although it is possible that seed mass variation might be selected because it increases dispersal effectiveness, several factors - the consistent within-cone positional variation in seed mass, wing area, and wing loading; the dependence of seed mass, wing area, and wing loading on cone size; and the positive relationship between seed mass and wing area - suggest that variation is caused by constraints by scale size.