Thursday, 19 April 2012

Mugwort sex

Mugworts sound like something out of Harry Potter, maybe a sort of school for Muggles, but no, they're plants, and they have interesting sex lives.  
The mugwort genus Artemisia is a large one, with about 400 species, according to Mabberley's Plant Book (Mabberley 2008).  The genus belongs in the Asteraceae (sunflower family) and within that family it's classified in Tribe Anthemideae, along with chrysanthemums, cotulas, tansy, and yarrow.  Many Anthemideae have aromatic leaves, for example chrysanthemum leaves give the evocative smell of florist's shops.
Yarrow, Achillea millefolium.
 Artemisia is no exception.  They're highly aromatic, like the soft silver A. arborescens that's grown as a low hedge in old New Zealand gardens.  A. absinthium is wormwood, which contributes mostly flavour to absinthe and A. dracunculus is the herb tarragon.
Silvery Artemisia arborescens at Makara, New Zealand.
Artemisias are wind pollinated, and this makes their flowers rather different from many in the family. 
Like the rest of the Asteraceae, Artemisia flowers are tiny florets clustered together in flower-like heads.  In many tribes, there are two types of florets: rays, the petal like outer ones, and disk florets, the tubular inner ones.  In Artemisia, the rays are very reduced, especially the corolla, which instead of being a long strap-like structure is just a very short tube that surrounds the style.
A typical daisy capitulum showing disk florets (top left and centre) and a ray floret (right)
 In many Asteraceae, ray florets are female, that is they have no stamens, while the disk florets have both an ovary and stamens.  However in the disk florets, the five anthers are joined in a tube that surrounds the style, and the pollen is swept out of this tube by the stigma as the style elongates.  Thus, even if the disk florets function purely as males, they have to retain a functioning style and stigma to sweep out their pollen.
Female ray floret (left) and hermaphrodite disc floret (right) of Artemisia vulgaris (from Garnock-Jones 1986).  Note the pollen-sweeping brushes on the ends of the hermaphrodite stigma, and the reduced corolla of the female floret.
Insect-pollinated flowers can combine male and female functions easily.  Their male function is dispersing pollen, which is done by attracting an insect such as a bee.  Their female function, receiving pollen, is also achieved when an insect visits.  Sometimes a single visit is enough, but in other plants the two functions are separated in time and two or more visits might be needed.
Wind pollinated flowers have a problem.  Their best strategy for dispersing pollen is for the flowers to be high up and facing downwards, whereas their best strategy for receiving pollen is to be low down and facing upwards, to receive the pollen as it floats down from above.  This is thought to be the reason why many wind-pollinated plants have separate male and female flowers.
Some wind pollinated daisies have separate male and female heads, but in Artemisia vulgaris they're combined, with outer female ray florets and inner hermaphrodite disk florets.  At flowering, the heads hang down, so the pollen can simply fall and be carried away by the breeze.  But the outer female flowers are also hanging down, which is not ideal.  They get around this handicap by having styles that curve around to point upwards on the outside of the head, a much better position for catching pollen.
Several flower heads of Artemisia vulgaris, showing the stigmas of ray florets curving around the outer bracts to face upwards, while the central disk florets disperse their pollen downwards.
 Both kinds of floret set fruit, but the female florets set more fruit, typically 2-3 times as many as the hermaphrodites.
Overall, the investment in male and female structures is quite uniform throughout the populations.  This means there is no tendency for some plants to produce more female florets than others do.  However, when it comes to fruiting, it's clear that the larger plants, those that are taller and have more stems, produce more fruit per 100 florets than small plants do.  Even though their flower ratios are much the same, the smallest plants produce very few fruits.  We don't know if they also father very few fruits, but given that they provide a short platform to disperse pollen from, that seems possible.  However overall, the larger a plant is, the higher its fruit output, i.e., the more female it appears to be.  Even with quite low percentages of florets producing fruits, a large plant can produce 3–5 million of them in a season.
The significant relationship between increasing fruit output (expressed as femaleness where a score of 1 is an exclusively female plant and 0 is an exclusively male plant) and plant height (left) and number of stems (right) in a population of A. vulgaris from Denmark (from Garnock-Jones 1986)
Other species have different strategies.  The coastal A. maritima has lost its ray florets, retaining only the hermaphrodite disk florets, whereas in A. campestris the disk florets are strictly male because they never set fruit.
Later in the season, when it's time to disperse the one-seeded fruits, the flower heads have turned upright.  The fruits have no plumes for wind dispersal, so maybe they need a vigorous shake from a gust of wind to dislodge them, a strong enough gust to blow them a short distance away from the parent plant.  If the heads stayed hanging, the fruits might simply fall to the ground underneath.
Heads of A. vulgaris at fruiting time; their stalks have straightened and now hold the heads erect.
Artemisia shows that even when plants are hermaphrodites (or cosexual to use the specialist term), individuals can still vary in their reproductive functions.  That's one of the ways that plant sex is interesting.  New Zealand botanist (and my PhD supervisor) David Lloyd used another member of Anthemideae, the largely New Zealand genus Leptinella, to work a lot of this out in the 1970s.
Garnock-Jones, P.J. 1986.  Floret specialization, seed production and gender in Artemisia vulgaris L. (Asteraceae, Anthemideae).  Botanical Journal of the Linnean Society 92: 285–302
Mabberley, D.J. 2008.  Mabberley's Plant Book (3rd ed.). Cambridge University Press.

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