Hilum

We’ve all heard of a “hill of beans”, but did you know that beans have hilums?

Cowpeas with visible hilums (Toby Hudson)

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Beans, peas, and other legumes produce fruits, called pulses, in pods. If you look closely, you can see where the seed attaches to the pod. Once the fruit or seed is mature, the pod opens along a seam, which means they are dehiscent. After the pod opens, the seeds fall to the ground where they are protected by a hard, water-resistant seed coat.

Bean seed (UC Davis)

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Seed coats have scars. When the seed separates from its pod, one scar is formed. This scar is called the hilum. On beans, the hilum is called the “eye”. Another scar, called the raphe, is a seed’s bellybutton. This is the scar that forms when the seed was separated from its placenta, within the pod. If you look even closer, you can see a tiny opening, called the micropyle, at one end of the hilum. This opening is where water is absorbed to allow germination to occur. 


Chestnuts have hilums, too.


Now you know.