Searching the kitchen for a true aril

Wattle (Acacia) seed with white aril

Whenever I see the word aril, I think of ants and wattle seeds. The aril is the variously coloured, wormy piece that hangs off the end of your typically black or brown hard-coated Acacia seed. 

It is much sought after by certain ants, who drag it, and the seed to their nest. The ants consume the aril, and should luck favour the seed, it can grow into a new wattle.

That's one particular kind of aril and one particular use and relationship. In its broadest sense, and we'll get to that, an aril is any fleshy appendage or outer layer on a seed coat. 

In kitchens, you'll find arils (in this sense) around the seeds of pomegranates, passionfruit and lychees. Those slippery, rubbery layers. The substances are different in composition and colour, and as it turns out, in origin.  

Passionfruit with seeds covered in aril

Neville Walsh suggested this topic to me after wondering if the goop around the seeds in a passion fruit were of the same origin as the slippery red surface on the seeds of a pomegranate. He assumed so at first, but was aware that the pomegranate gelatinous layer is called a sarcotesta rather than aril. 

Pomegranate seeds covered in sarcotesta

Both the sarcotesta and aril are formed from the seed coat, so they a similar kinds of things. I turned first to The Kew Plant Glossary, which applied the term aril to pretty much any additional structure on the seed formed from the seed coat.

Yet traditionally, and for pedants, the aril is narrowed down to just the expansion of what is called the funicle - the thread connecting the placenta to the ovule (think umbilical cord). The term arillode, or false aril, is sometimes used for an aril-like layer not deriving from the placenta. 

You will find - if you are into this sort of thing - that other word sarcotesta used for pretty much the same thing. Examples cited include the fleshy outer (often red) layer of cycad seeds and that rubbery surface of a pomegranate seed.

Cycad seed with red outer layer (sarcotesta)

Enid Mayfield, in her highly recommended Illustrated Plant Glossary (informed by Neville Walsh on matters of botanical complexity), defines an aril as a fleshy appendage on a seed, 'of various origins'. She does narrow down with a 'specifically', referring there to the funicle definition (and mentioning ants and - indirectly through reference to the pea family - wattle).

The sarcotesta, according to Enid, is the outer fleshy layer of a seed. There is some subtlety around this being a 'layer' in the single coat of a conifer seed or an 'integument' in a flowering plant seed. But basically fleshy rather than hard, and not explicitly linked to the funicle. The pomegranate seed is presented as an example.

So it seems that you can use aril for any fleshy outer covering or appendage of a seed, sacrotesta for those cases where the outer layer has become gelatinised or fleshy, and aril sensu stricto (in the narrowest sense) for the ant and wattle example. 

CC BY-SA 2.0 (https://en.wikipedia.org/wiki/Aril)

If you are looking for another aril in that narrower sense, you might also find that in your kitchen. The spice mace is the ground-up lacy expanded funicle of a nutmeg seed. It's a beautiful red colour when fresh, as the Creative Commons photo above shows. 

While there is no mace in our pantry, I did find a can of lychees, or should I say, 'lychee arils'. 

The 'aril' of a lychee seed (with hard-walled seed and outer skin of fruit removed)

That creepily textured flesh of a lychee is apparently part funicle and part out integument of the seed coast. So it's an aril in both senses of the word. 

Passionfruit pulp, however, is generally considered to be 100% funicle. A true aril, and if I were an ant, I'd be seeking out Passiflora rather than Acacia.