Late summer flowers

pollinators, seed dispersal, and other end-of-season floral stories

One sunny day, I sat in a small meadow in the Kowee Creek drainage, just taking in the peacefulness and a snack. Some small, red-eyed flies found me and crawled over my arms, dapping at the skin with their nonbiting mouthparts—possibly tasting a bit of salt. Several of them had substantial amounts of yellow pollen on the thorax. All over the meadow, little white flowers showed their heads among the mosses and sedges. I knew these to be flowers of swamp gentian (Gentiana douglasiana). They were about a centimeter across, with odd little pleats in between the five white petals. Because they were the only flowers to be seen, I suspected that the flies are among the pollinators of those flowers—although I did not actually see them visiting the flowers.

What a contrast with the goldenrods that bloom along the road above Eaglecrest. There, on another nice summery day, the golden inflorescences were often thronged with visiting insects: flies of various sizes, a wasp or two, hoverflies, and bumblebees.

As I traipsed through another meadow, this time on the Spaulding trail, I saw tiny white flowers that were borne on branching stems that reminded me of wee candelabras. Looking more closely, I saw that each flower, only a few millimeters across, looked like miniature flowers of the swamp gentian flowers I’d seen earlier. Could this be the same species? The answer turned out to be Yes. By diligent searching, I found a few plants that bore single large flowers on one stem and acandelabra of tiny flowers on another. So I learned that this species produces flowers that vary two- or three-fold in flower size, apparently depending on how many flowers are borne on a stem. This suggests that there may be a trade-off between size and number of flowers per stem: more flowers, then small size; one flower, then large size—as if the plant had made some hormonal decision about how to allocate its floral displays.

In the muskeg ponds, buckbean flowers were finished and the seedpods were maturing. Some had opened, shedding their seeds onto the water surface. The seeds floated nicely, but the opportunity for dispersal by water was limited, because there was no surface-water connection among ponds. That would mean that all the buckbeans in one pond are closely related to each other—the seeds being genetically at least half like the mother plant. However, insects probably transfer pollen among plants in different ponds, increasing the genetic diversity within each pond while reducing the potential differentiation among ponds.

The big tall fireweed that is so common here is producing lots of fluffy seeds to blow on the breezes. That, however, doesn’t work when rain flattens those fluffy parachutes (into nets that are beautiful with captured raindrops). I wonder how many of those seeds get airborne. Landing in a suitable place is always a lottery, but getting into the lottery in the first place is a necessary step. Rain gets in the way of seed dispersal for these plants.

Some of the tall fireweeds still had some flowers and even buds at the top of the stem. Although most of the flowers are about the same size, on one plant I noticed flowers that were only about half of the normal size. Weird! I have read that drought conditions can lead to production of significantly smaller fireweed flowers, but that would not explain this single small-flowered plant. Then I happened to peer more closely within a normal flower (shame on me for not having done this sooner!) . First, I was reminded that fireweeds produce blue pollen; although blue pollen is known from a few other species elsewhere, it seems to be unusual here. That simple observation raises the immediate question: Why blue?

The flowers mature in sequence from the lower part of the inflorescence to the tip, where buds can still be found when the older, lower flowers have already produced mature pods. Within each flower, the pollen-producing anthers mature before the female-receptive surface (the stigma). The lengths of the male phase and female phase of each flower varies; the duration of the male phase is longer early in the season, when there are fewer female-phase flowers available to receive pollen and mating opportunities are few.

Fireweed is self-compatible, meaning that pollen from the same flower, or another flower on the same plant, can be effective in fertilizing seeds, although outcrossing, with pollen from different plants, also occurs. Perhaps because seeds can be produced either by selfing or outcrossing, fireweed flowers seem to produce pods very successfully. Furthermore, as a flower ages, the lobes of the (unpollinated) stigma bend back and down, bringing the receptive surface closer to the nectaries, where a visiting bee would be likely to forage, perhaps bringing in pollen and thus increasing the probability of fertilizing seeds.

The flowers have several means of increasing the probability of outcrossing. The difference in timing of male and female parts reduces the likelihood of pollen landing on the stigma of the same flower (but not between flowers on the same plant). The flowers start to close about four hours after pollination, well before any seeds are fertilized (it takes many hours for sperm from the pollen to reach the eggs in the ovary). The flowers close faster if outcross pollen is deposited than if self-pollen lands on the stigma (slower closing of selfed flowers leaves more time for outcross pollen to arrive), and faster if there is lots of pollen deposited than if few grains are deposited.

Photo by Bob Armstrong

Footnote: in a recent essay I commented that I had found marsh felwort only in two places. No sooner than I had written that, I found lots of the little blue, starry flowers in Cowee Meadows, mostly over toward the creek. Although this plant is an annual, it’s probably been there all along; I simply was not there are the right season.

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