Botanical Illustration: Exploring the achene fruit type May 2nd 2014

Story posted: Friday, 2. May 2014 by Lizzie Harper

Natural science illustration and natural history illustration require you to understand both what your subject looks like, and also the correct words needed to scientifically describe it.  Last week my blog was about fruit type definitions, inspired by some work I did for Rodale's 21st Century Herbal by Michael Balick.  Whilst getting my head around the terminology of fruit types, I realised there's scope for a whole blog about the seemingly humble ACHENE.

The ACHENE is "a small dry indehiscent single-seeded fruit" (Flora of the Birtish Isles, Clapham Tutin and Moore). So far so good.  But under this umbrella definition is an enormous amount of variation.  The fruit of the buttercup and crowfoot is often referred to as a "typical achene"; a fruit containing a single seed, where the fruit wall has hardened around the seed inside and thus covers it like a coat (without actually sticking to it).

 

Lizzie harper botanical illustration of a buttercup

Creeping buttercup

INDEHISCENT means the fruit doesnt open to disperse the seed at maturity.

Achenes only contain one seed, and each achene is formed of this single seed produced by one carpel.  A CARPEL is one of the units that the GYNOECIUM is made from.  And a GYNOECIUM?  That's the female parts of a flower; so it's the ovary (which, when fertilized grows into the seed), the style, and the stigma.

Sometimes, an achene can have wings, and in this case it's called a SAMARA.  These are extensions of part of the tissue of the wall around the seed which grows outwards and flattens.  The purpose of these wings is to aid in wind dispersal, which explains why it's mostly tree species who develop them.  The wings can grow on both sides of the seed, as in the case of the elm, bush willows, and hoptree.

Lizzie Harper botanical illustration of elm samara

Elm

Samara can also have one wing or extension, leaving the seed at one end of the wing.  This is the case for maples (where these single winged samara are paired into the instantly recognisable shape), and for the ash.  

Lizzie Harper Botanical illustraation of sycamore samara

Sycamore

The benefit for a tree of this design is that the seed "auto-rotates" as it falls, ensuring a clean and distant dispersal from the parent tree.  When something auto-rotates, it means that it goes on rotating as it falls through air in a steady fashion (as unpowered helicopter baldes would do).  Try it next time you have a maple or ash key in your hand, it's a nifty piece of aerodynamics.

Lizzie Harper botanical illlustration of ash keys copyright Jersey Post

Ash keys (copyright Jersey Post)

Asteraceae are plants made of two types of flower, ray and disk florets.  A common example is the daisy, and the dandelion.  Each yellow "dot" in the centre of a daisy is a single yellow disc floret; each white external "petal" is a ray floret.  Asteraceae species have different selections of ray and disc florets according to species.  Across the entire botanical family (which used to be called Compositae), CYPSELA are produced as seeds.

Here we encounter a slight hiccough, as some botanists claim that although cypsela are similar to achenes, they are, in fact, a different thing.  This is because they're produced from a compound inferior ovary with one locule.  This basically means they're produced by several ovaries which are joined together, each one of which is made of one chamber, and the whole lot is sited below the petals and rest of the female flower parts.

However, most botanists agree that a cypsela is so very similar to an achene to permit it to be out under the same umbreall term, hence it appearing in this blog.

The dandelion seed, with its distinctive parachite, is a cypsela.  The fluffy part is made from part of the calyx, whose tissues have evolved into the intricate flying machine which attaches to each seed.

The sunflower seed is another example of a cypsela; the white or striped cover of the seed is the wall of the cypsela fruit.  Dissect a sunflower seed; it's easy to see the hard outer coat and the seed inside; closely surrounded by -but bot attached to- the fruit's wall.

Lizzie harper botanical illustration of a sunflower

Sunflower

Within each rose hip, hidden amongst the hairs and the flesh, are a few achenes.

 

Lizzie harper botanical illustration of a dog rose hip

Dog rose rose-hip

For me, the most remarkable fact I found about achenes was that the external seeds that pepper the outside of a strawberry are all individual achenes.  Each one encases a single seed.  The delicious strawberry that we eat is, in fact, not a fruit at all (each of the strawberry pips IS a fruit);it's merely the sweet tissue which bears the achenes.

 

Lizzie harper botanical illustration of wild strawberry

Wild strawberry

Other achene-bearing plants include the tall anenome, cannabis, buck-wheat, crows-foot, lesser celendine, globe artichoke, spearwort, fleabane, zinnias, chrysanthemums, lettuce, marigold, echinacea...  Considering how many plants bear achenes, I reckon it's worth knowing what one is, and hopefully this blog has helped to explain just that.

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