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Natural North Dakota

Milkweeds and Fruit Production


You may recall that last week’s Natural North Dakota topic was about milkweed’s interesting pollination mechanisms.  I am still thinking about the common milkweed, but this time about fruit and seed production.    

If you are at all familiar with plants, you know that if all goes well, each flower should produce a fruit. An apple blossom should produce an apple.  Each of those flowers in the inflorescence of a chokecherry plant should produce a chokecherry.  And of course, the fruits should contain seeds to produce the next generation.  

The common milkweed is different.  The common milkweed produces its flowers in dense clusters of somewhere between roughly 20 to over 100 individual flowers.  What we call a milkweed pod is the fruit of a milkweed plant.  But we do not see between 20-100 milkweed pods on a plant when autumn rolls around.  There are often only 2-4 pods produced on a plant, each loaded with seeds.  

It could be that only a few of the flowers are actually pollinated and as a result, only a few pods develop. Or perhaps most of the flowers were pollinated and for some reason only a few develop.  It is not fully understood, but studies on milkweed reproduction have found that most of the flowers are pollinated, but somehow the plant can assess how much energy it can expend in fruit and seed production and allocate that energy accordingly.  An energy stressed plant will produce fewer pods and seeds than one with abundant energy reserves.  

That might seem strange, but in milkweeds, apples, acorns, and many other species, fruit and seed production is thought to be related to how much of the plant’s energy resources it can “afford” to use in reproduction.  There are tradeoffs between reproduction and other aspects of energy use.  For example, the plant might be better off by putting some amount of energy into root growth instead of seeds.   Somehow plants have the ability to adjust or change the allocations of energy from year to year.  

So if you see some common milkweeds, make a point to observe them over the next couple months to see what happens with respect pod and seed production.  They may not have brains, but they are more responsive to physiological and environmental cues that we can fully explain.

-Chuck Lura

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