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Petal-less flowers at the Arnold Arboretum

by William (Ned) Friedman, Director of the Arnold Arboretum
April 18, 2017

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Petal-less flowers at the Arnold Arboretum

The beginning of spring flowering has fully launched at the Arnold Arboretum. And this is the perfect weekend to stroll through the collections and see what is on offer. While most of the show revolves around the myriad forms and colors of petals, step off the main path and have a look at a few species whose flowers lack petals (as well as sepals). Their beauty is subtle, but cumulatively can add up to an amazing effect. On offer this year (spring conditions were just perfect) is Euptelea polyandra (Japanese euptelea; 1610-77*C), just off Linden Path. The Arboretum has just one specimen of this shrubby species and it is in full bloom right now. The crimson stamens (pollen-producing organs) are dangling by the boatload (upper image), and if you look carefully, you can see fuzzy white structures (like miniature bottlebrushes), stigmas, to catch the pollen.


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Right around the corner, head for the wonderful old katsura tree (Cercidiphyllum japonicum; 882*A) along Meadow road. Step away 50 feet and you will see an intense and subtle red glow to the entire tree – the result of thousands of flowers with their brilliant red gynoecia (female part of a flower) glowing in the sun (lower left). This particular tree is one of the oldest in the collections and also one of the first plants to make its way from Asia to the Arnold Arboretum (in 1878; more info here). Katsura trees are dioecious (individual trees either produce pollen-bearing flowers or seed-producing flowers). Nearby, you can find a male tree with clusters of stamens (178-87*C; lower right) swaying in the breeze.

The evolution of petal-less flowers is typically associated with transitions from animal (typically insect) pollination to wind pollination. With no need to attract a pollinator, natural selection has reduced these flowers to just the essential sexual parts: pollen making organs and seed producing organs. And without animals to make an efficient “beeline” to other flowers, petal-less species typically produce enormous quantities of pollen – hence the large number of prominent stamens. Euptelea and Cercidiphyllum are perfect examples of this syndrome.

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