Sunday, August 24, 2014

Goldenrod and the Composites

When most other flowers have come and gone, this is the season for the many species of Goldenrods, their tall yellow plumes waving in the breeze.  I was watching bumblebees and two different kinds of wasps probing the flowers of a goldenrod by my driveway the other day when I became curious.  Most goldenrods contribute to the blaze of yellow you see at this time of year with an extravagant show of flowers, but the flowers themselves are small.  About how many flowers are there?  I plucked a branch--one of perhaps thirty in the cluster, and carefully counted the tightly-packed flowers on it: eighty.  If the branch I counted was fairly typical, the whole plant had 2400 flowers.  

Goldenrods (Solidago) attract bumblebees and wasps, among other insects.

Focusing on a single branch, we see dozens of things that look like flowers.

I counted 80 flower heads on a single branch.

But not so fast.  Looking closely at a few flowers, I count a dozen or so petals.  If you've looked at a lot of flowers, you realize that a great many have five petals, monocots such as lilies have three or six, mustards have four, but none have more than six petals.  Coming across a flower with a dozen petals signals us that we are NOT looking at a flower--we are looking at a cluster of flowers packed so tightly they appear as one: a flower head.  You will find this in a variety of familiar plants, from goldenrod to dandelion to aster to daisy to sunflower--all members of a single large family of flowering plants: the Composites. That means my goldenrod has about 2400 flower heads, not individual flowers.

The Composites, aka Aster Family or Asteraceae, are hugely successful in their variety.  Gleason and Cronquist's Manual of Vascular Plants of the Northeastern United States and Adjacent Canada (2nd edition) lists 147 genera with too many species for me to count, devoting 112 of its 900-odd pages to descriptions of this one family.  This success is presumably due in part to their distinctive feature: the flowers of composites are tiny things, but they are massed together in much larger flower heads.  In effect, a composite multiplies its chances of attracting pollinators by having its tiny flowers in larger, more-visible masses

Composites I noticed while walking the dogs.
(If I don't know exactly what they are, how do I know they're Composites??)

Black-eyed Susans are a familiar member of the family.

This is not the Common Dandelion (Taraxacum officinale), 
but a distant sister, Fall Dandelion (Leontodon autumnalis)

 Instead of the barbed-arrow-shape of common dandelion leaves,
fall dandelion has leaves with an outline like  some Medieval weapon.

On the other hand, fall dandelions spread their offspring on the wind, the same way common dandelion does.

 Aster, one large genus of Composites, gives its name to the whole family: Asteraceae.

Some of these flower heads, like those of dandelion and hawkweeds, simply look like huge masses of petals.  But others have flower heads that look for all the world like single flowers: a ring of  "petals" surrounding a colorful center.  That's because their flowers are differentiated into two types: tiny "disk flowers" and long, strap-like "ray flowers."  Careful examination will show that a ray flower is has its five petals joined together in a single long strap, and you can prove that it is itself an entire flower by finding its pistil--the part of the flower that accepts the pollen and makes the fruit. 

The artful devotees of this strategy form a family of flowers that includes many familiar species that brighten our gardens and colors our roadsides and fields--that, in fact, rules the early summer months.

Returning to my goldenrod.  Though made of many flowers, even the flower heads of goldenrod are tiny--perhaps 5 or 6 mm tall.  By massing these heads into even larger clusters, the goldenrod does one better: heads themselves are further massed, making a cluster that may be a foot tall and wide, creating a huge display.  Of how many flowers does this display consist?  Each tiny head is made up of even tinier flowers of both kinds: disk and ray.  The dozen-odd ray flowers surround a smaller group of disk flowers.  Suppose the typical head has 20 flowers in total.  This would make the total 20X2400=48,000 flowers on a single plant!.  And every one of them is ready and able to make a seed.  

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