Monday, July 9, 2018

The odd seed baskets of carrots

Flowers of the carrot, Daucus carota, are borne in a flat-topped
inflorescence called a compound umbel.  Ignore the
foliage in this photo, as it belongs to a neighboring potato plant.


Most people don't notice the elegant inflorescences of the carrot plant (Daucus carota).  If you do, it usually means you've waited too long to harvest the edible, orange taproots.  If you have seen them, you might have noted the resemblance to Queen Anne's Lace, which is in fact a wild relative of the cultivated carrot. 

The individual white flowers are borne in small, flat-topped clusters called umbels, for their resemblance to little umbrellas.  The umbels, moreover, are grouped into a more compound structure, creating a large, flat-topped display for the tiny insects that will feed on the flowers and disperse their pollen.  The carrot family, Apiaceae, used to be called the Umbelliferae, after this characteristic inflorescence structure.  Celery, coriander, celantro, parsley, dill, fennel, and a host of other plants useful for nutrition and seasoning, belong also to this family.

Even fewer people have noticed the strange contortions these inflorescences undergo during their development and the maturation of their seeds.  As the young flowers begin to form, they are hidden and protected within the in-turned inflorescence branches and a series of spiky bracts.



As the flowers mature, the branches of the inflorescence expand and bend outwards to form the compound, flat-topped blooming structure.

The surprise comes as the flowers wither and their ovaries mature into the tiny, one-seeded fruits, that we superficially take for bare seeds.  The fruits of the  carrot and other members of the Apiaceae (sunk by some into the Araliaceae) are technically schizocarps, as they consist of two single-seeded units that split apart as they mature.  The fruit wall is thin, and dries into a hard outer layer on the seed, and so is unnoticed. 

As this ripening process proceeds, the branches of the inflorescence bend inwards again, bringing the fruits inside of  what is now a basket-like structure.  It is likely that this phenomenon is an adaptation for protecting the fruits from herbivores as they ripen, but I've not been able to find any literature to verify this.  The basket may also serve as a giant salt-shaker like structure that sways back and forth in the wind, helping fling the seeds away from the parent plant.  A similar structure has been noticed in the related genus, Conopodium, and is likely to be found in other members of the family.





As the young flowers develop, they are protected within the in-rolled
branches of the inflorescence.
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As the ovaries of the flowers develop into the dry fruits known as schizocarps,
they are drawn inside of a basket-like structure, by the inward curving
 inflorescence branches.
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