Seed size variability: from carob to carats

Area of Science:

• Botanical studies
• Metrology
• History of science

Background:

• Seeds have historically been used as weights due to perceived uniformity.
• The carob seed (Ceratonia siliqua) is famously associated with the carat weight standard.
• The constancy of carob seed weight and the origin of this belief are questioned.

Purpose of the Study:

• To investigate the natural weight variability of carob seeds.
• To compare carob seed variability to other plant species.
• To explore the role of human perception in the carob seed weight myth.

Main Methods:

• Sampling and weighing seeds from a collection of carob trees.
• Literature review of seed variability across 63 plant species.
• Conducting a perception experiment on human ability to discern carob seed weight differences.

Main Results:

• Carob seed variability (CV=23%) was similar to the average of other species (CV=25%).
• Human observers could detect weight differences of approximately 5% by sight.
• Pre-metrication carat weight standards also exhibited around 5% variability.

Conclusions:

• Carob seeds do not possess unusual natural uniformity.
• Human perception and selection played a significant role in establishing the carob seed as a standard weight.
• The myth of the carat's origin is attributed to human intervention rather than natural constancy.