Word of the Week: Aril

aril [ ar-il ] noun: an extra seed-covering, generally fleshy and brightly colored, especially one that develops from the funiculus or seed stalk

Click on images on this page for larger views and captions.

If you have a magnolia tree on or near your property, then you may be familiar with the cone-like fruit it produces in summer. When the follicles split open, they extrude what appear to be bright red or orange seeds. However, the seeds themselves are actually hidden under oily coverings called arils. Unlike a seed coat (integument) that develops from the outer cell layers of an ovule, an aril grows from the point (funiculus) at which the ovule (seed) attaches to the ovary wall.

As flowers have coevolved to attract pollinators, fruits have coevolved to attract dispersal agents–in this case avians, like song birds and woodpeckers, and mammals, like squirrels and opossums. The bright colors and sweetness of arils entice the animals to this food source. They digest the arils and regurgitate or excrete the bare seeds (endozoochory), ready for planting and hopefully transported far enough away from the mother tree. (People who harvest seeds like magnolia usually soak the seeds in water first, then rub them against a hard surface to remove the arils to prepare the seeds for sowing.)

Besides dehiscent fruits like magnolia and Euonymus americanus (strawberry-bush), arils also are found in drupes like Myristica fragrans (nutmeg), and in berries like Passiflora (passionfruit). The glossy arils of nutmeg, which appear like tracery over its seeds, produce the spice mace when dried. Many people prefer the sweeter pulp (the yellow to orange, membranous, juicy arils) of tropical Passiflora edulis or P. edulis f. flavicarpa (purple or yellow passionfruit), which are commercially available, over our native P. incarnata (maypops).

Female strobilus of Taxus brevifolia (pacific yew). Each red, cup-shaped, berry-like aril holds one seed.
Photo courtesy of VA Tech Dendrology

An unusual aril occurs in the female strobilus of Taxus (yew): a red, berry-like cup that holds a highly poisonous seed. Some debate whether this fleshy cup is actually an aril or a sarcotesta derived from the integument [as some call the juicy sacs of pomegranate, although other sources call them arils] or a fused pair of strongly swollen leaves (Dörken et al., 2019). In any case, birds are attracted to and can consume the non-toxic arils and disperse the seeds in their droppings without poisoning themselves.

References

Dörken VT, Nimsch H, Rudall PJ. 2019. Origin of the Taxaceae aril: evolutionary implications of seed-cone teratologies in Pseudotaxus chienii. Annals of Botany. 123(1): 133–143. DOI: 10.1093/aob/mcy150.

Morton, J. 1987. Passionfruit. pages 320–328. In: Fruits of warm climates. Center for New Crops & Plant Products.

Silveira S, Dornelas MC, Martinelli AP. 2016. Perspectives for a Framework to Understand Aril Initiation and Development. Frontiers in Plant Science. 7:1919. DOI:10.3389/fpls.2016.01919.

This entry was posted in Illustrated Glossary, Word of the Week, WoW and tagged , , , , , . Bookmark the permalink.