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Annis Richardson
University of Edinburgh, UK
Pecans and walnuts may just look different to other “nuts” because they don’t share the same developmental origin.
Not everything we call a nut is the same thing. For example, hazelnuts and chestnuts are “true nuts” formed when a fertilised flower’s ovary wall swells and hardens, and these nuts don’t open when they are mature. Peanuts are actually related to peas, and they grow in pods underground.
Walnuts and pecans are called “drupaceous nuts” – they have a fleshy outer layer, then a hard inner layer called the endocarp. The edible walnut is actually the seed inside this.
The shape of the walnut arises from mechanical differences between the inner tissues and the outer, drier, rigid tissues. These differences mean that the surface of the seed deforms, making the wrinkled shapes – it’s all a matter of physics. This means there may be no advantage to walnuts and pecans having this wrinkled appearance!
Madelaine Bartlett
University of Cambridge, UK
The convoluted, fleshy bits of walnuts and pecans that we eat are modified cotyledons (baby leaves) that have a nutrient absorption function, rather than a photosynthetic one.
In other words, they are leaf-like organs that absorb all the nutrients from the endosperm (a nutritious tissue that feeds developing plant embryos and seedlings), rather than making their own through photosynthesis. This adaptation is relatively common – the single cotyledon of grass embryos, for example, serves the same purpose.
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