Seed size variability: from carob to carats

Abstract

The seeds of various plants were used as weights because their mass reputedly varies so little. Carob (Ceratonia siliqua), which has given its name to the carat, is particularly famous in this regard. But are carob seeds unusually constant in weight and, if not, how did the myth arise? The variability of seeds sampled from a collection of carob trees (CV=23%) was close to the average of 63 species reviewed from the literature (CV=25%). However, in a perception experiment observers could discriminate differences in carob seed weight of around 5% by eye demonstrating the potential for humans to greatly reduce natural variation. Interestingly, the variability of pre-metrication carat weight standards is also around 5% suggesting that human rather than natural selection gave rise to the carob myth.

Figure 1

Seeds from five pods (rows) belonging to three different trees (place or origin or variety shown).

Figure 2

(a) Seeds from hermaphrodite (hermaph.) carobs are lighter than those from females (ANOVA: F_1,26=5.5, p=0.027) but have similar variability (box plot shows median, 1 and 1.5 times the interquartile range, and extremes); (b) Coefficients of variation for 63 species collected from the literature with the overall mean (dotted line), filled square, carob; open squares, other Fabaceae; open circles, non-Fabaceae species; (c) CVs of individual carobs decline with mean seed mass (regression: F_1,26=15.9, p=0.0005, R²=0.34), filled, hermaphrodite; open, female.

Figure 3

(a) Proportion of observers who correctly identified the heavier of a pair of seeds as a function of their CV (logistic regression: ${\chi }_1^2=19.7$, p<0.0001). (b) The distribution of seed weights in the total sample. (c) The distribution of carat weight standards worldwide prior to 1907 (when the metric carat was adopted).