Sunday, June 7, 2015

Redring Milkweed (Asclepias variegata)

April 30th – May 18th, 2015. The Redring (or White) Milkweed (Asclepias variegata) is my favorite milkweed. I knew that there were several plants both in the open against, and in the edge of deciduous woodlands along the trail from the picnic area to the Old Fort. I’d found them several years ago and was curious as to how they were doing.

Milkweeds produce umbels, a flower head in which the flower stalks are of the same length, so that the flower head is rounded like an umbrella. The stalks (pedicels) that support the florets of the Redring Milkweed are stiff so that the florets are arranged in a ‘tight’ umbel. 

I had checked the sites several times but it wasn’t until April 30th that I found them.

They were already about a foot tall and beginning to set flower buds.

Closer views of the flower umbels. Each plant only set one umbel this year although a couple had set multiple umbels a few years ago. One plant started to set a second umbel (lower image) but this did not develop.

The umbels on May 6th. The buds on the younger umbel (upper image) were more angular in appearance compared with those on the older umbel (lower image).

On May 11th, the florets had opened.

Asclepias variegata gets one of its common names, ‘Redring,’ from the red ring at the base of the hoods. This is a diagnostic characteristic for this species.
The anatomy of milkweed flowers is intriguing in several respects; excellent detailed diagrams of their anatomy may be found here.

This is a simplified illustration of the visible parts of the Redring Milkweed floret. The stigma and anthers are fused with a stigmatic disc on top to form a gynostegium. The gynostegium is surrounded by hoods with horns that fold over its stigmatic disc.
Fertilization of milkweed flowers is more complex than for most other flowers. The pollen is not released loosely from the anthers but is contained in sacs called pollinia (singular = pollinium); two pollinia are connected by a ‘thread’ composed of a translator and corpusculum. In order to fertilize the flower, an insect must hook the translator, pull the pollinia out of the anther and drag them over the stigmatic disc so that the pollen is released directly onto the disc. This is why milkweed flowers always look so clean. 

The most frequent ‘local’ milkweeds that we see – the Butterfly Milkweed (A. tuberosa), the Clasping Milkweed (A. amplexicaulis), and the Green Comet Milkweed (A. viridiflora) produce seedpods readily. We don’t see Redring Milkweeds frequently and, when we do, we rarely see them with seedpods. 

The hooks on the flowers of many milkweed species do not fold tightly over the stigmatic disc, a factor that probably allows insects to fertilize flowers fairly easily. The Redring Milkweed flower is unusual in that the hooks are folded tightly over the stigmatic disc. There is ample space under the hooks for small insects to crawl in and snag the pollinia but they may not be big or strong enough to do so. Larger insects may not break through the hooks to snag the pollinia or may not be as interested in the flowers because they lack readily accessible pollen. In any case, a sighting of the Redring Milkweed is exciting and we check them to see if they are forming seepods. We’ve only seen two plants form seedpods; one was high on a cliff face and the second was destroyed before it matured.

I checked on the plants on May 26th. Four flower stalks were still attached to the plant, the first sign that a plant may be going to form seedpods. However, there was no sign of the thickening of the stalk, needed to support a developing seedpod. Sadly, these stalks were gone when I visited the plants next.


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