Electric flowers and platform plants

Many plants produce flowers as a way of attracting animal visitors that can pick up pollen and move it to another flower. Flowers come in an array of colors—all the wavelengths we can see plus UV (which most humans cannot see). A yellow flower with a UV pattern is readily distinguishable from other yellow flowers—to the many kinds of animals (including insects and hummingbirds) that can see UV. That’s been known for many years.

However, flowers actually have (at least) two ways of enhancing their distinctiveness that humans generally cannot detect without special equipment. More recent research has found that plants can exploit another sensory system of animals. 

There is a natural electric gradient from the ground (negative) up into the atmosphere (positive). And around every plant and its flowers there is a weak electrical field. Flowers have a negative charge (like the earth they come from), but they bloom in positively charged air, creating a little electrical field. Around the flowers that electrical field is strengthened: the electric effect is best developed around edges, such as the rims of petals and the inner (sexual) parts of the flower. So the size and shape of the flower is emphasized and made even more distinctive.

Bumblebees and other insects can detect the presence and shape of the floral electrical fields and use the information to decide which flowers to visit. Bees detect the electrical fields with their fuzzy hairs. The floral electrical fields are weak, but they are strong enough to deflect the hairs (just a wee fraction of a degree) and set off neural signals that the bees can interpret. Bees’ antennae can detect the fields too but no neural signal is sent on. Experiments with artificial flowers (identical in color, shape, and size) with and without nectar rewards and with and without electrical fields around them showed that bees quickly learn to choose the rewarding, electrical flowers.

Further research revealed that hoverflies, which are also important pollinators, can also do this well. Their body hairs are deflected by small electrical fields and a neural signal is sent. The flies learn to read the signal and their efficiency and speed of finding floral nectar rewards increases.

This hoverfly can perceive electrical fields around the edges of the petals, the big white stigma, and the stamens of this fireweed flower. Photo by Bob Armstrong

Electricity also helps move pollen from floral anthers to insects, because insects have a positive charge and the flower a negative one. So loose pollen can actually jump small distances from anther to insect, even before the bug lands. If the plant is lucky, the insect carries the pollen to another flower. If not, a bee may groom the pollen off its body and packs it away in its pollen baskets to feed bee larvae.

Plants are commonly used as platforms for transmitting vibrational signals, usually among themselves. For example, treehoppers suck plant sap, often gathering in considerable numbers on plants. The treehoppers contract their abdominal muscles very fast, creating surface vibrations that move over the plant. We can’t hear them without the aid of special devices, but the vibes are picked up by the legs of other treehoppers. Those vibrations are like songs, varying in pitch and tempo, and are clearly interpreted by the receivers.

Some songs are used for courtship, drawing male and female closer together; a second male can jam the first male’s songs and decrease a female’s response, thus interfering with the courtship. Baby treehoppers (called nymphs) emit cries of alarm when danger is perceived; this elicits defensive behavior of the mother, who signals after evicting a predator, calming thenymphs. 

Lots of other insects (and spiders) can communicate with each other using plant vibrations. Many female insects use variation in male vibrations to choose the right species and the best male to mate with. It goes the other way too: males can use female vibrations to discriminate among females. Insects such as katydids make sounds we can hear, but they also make vibrational signals that indicate body size, and females prefer larger males. Leaf-cutter ants create vibrations when chewing into leaves; if it’s a really good leaf for growing fungi to feed the colony, the distinctive vibrations can recruit other foraging ants to exploit the good resource. The larvae of some insects use vibrations to attract others of the same kind or to keep competitors away.

A typical range of transmission for most of these vibrational signals is up to about two meters, although it can be longer for large insects and spiders. All vibrational calling is energetically expensive, and some studies have shown that an insect that calls a lot is not likely to live as long as one that calls infrequently. The host plants that provide the platform have different vibrational properties, so they differ in their signal transmission capacity. 

Some plant-based vibrations are not meant as communication among members of the same species. Vibrations produced by feeding, for instance, can be risky if it attracts predators—and lots of potential predators can track such vibrations. For example, a feeding caterpillar inadvertently gives vibrational cues to a predatory stinkbug and perhaps to parasitic wasps. In contrast, some butterfly caterpillars vibrate along with chemical signals to call in mutualistic ants to provide protection from predators. And there’s a spider that mimics the vibrational signals of the males of other species, to lure females of those species into hunting range for the mimicker.

Clearly, plants do far more than most of us ever imagined! That’s just a sample of sensory worlds that humans cannot experience directly. We miss a lot!

The stories of electric flowers and singing tree hoppers came mainly from a fascinating book about the sensory world of animals (An Immense World, by Ed Yong).

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Summer flowers

Most of us have favorites among the very showy flowers, such as the fireweeds, or white bog orchids, or columbine, or wild iris, or we look for uncommon species, such as frog orchids. These may be the stars of the show, but we may neglect some ‘lesser lights’ that are interesting in their own right. I’ve picked out just few of these here, simply because I’ve seen them recently on July walks.

In one of the meadows on the way up to Spaulding Meadow, the density of sundews is remarkable—there’s almost a carpet of round-leaf sundew (Drosera rotundifolia) over the mosses, and long-leaf sundew (a.k.a. great sundew, D. anglica) grows mostly on the muddy edges of pools. Sundews are insectivorous, supplementing what they can draw from their nutrient-poor habitat by digesting insects captured on the leaves (and they may have ways to avoid capturing potential pollinating insects). I noticed that very few of the sundews had produced flower buds at this time. Because flower (and eventually seed) production costs energy and nutrients, I wondered if these sundews were not capturing many insects to help fuel flower production. Was there a seasonal low in insect availability or maybe just not enough bugs to feed so many sundews or possibly (as found in one study) too much competition from spiders that want bugs too?  Or something else….??

Self-heal showing fringed lip and hood. Photo by Mary Willson

Along the road to the Salmon Creek powerhouse, the hiking group found common harebells and lots of a small, purple-flowered perennial plant called self-heal (Prunella vulgaris). It’s native across the northern hemisphere and introduced everywhere else. There are multiple flowers in each inflorescence. The flower has a fringed lower lip and an upper hood over the stamens and pistil, but in some cases the pistil extends out in front. Flowers with the pistil inside the hood tend to have bigger and fewer flowers, less pollen, less nectar, and lower visitation rates—and apparently less male function. Although the flowers may self-fertilize if few pollinators are available, they are primarily bee-pollinated.  I watched a bumblebee unsystematically visit nearly every flower on one inflorescence, poking its head deeply into some of them to get the nectar and passing quickly over others (perhaps the nectar had already been taken). 

Self-heal with bumblebee. Photo by Deana Barajas

Studies of self-heal in Japan have shown that the size of the flower in different ecological settings varies with average tongue length of the bumblebees in those settings: bigger flowers in areas with long-tongues bees. In other words, there are local adaptations of flower size to the abilities of the available bees. Other factors, such as altitude or robustness of the plant, did not account for the observed correlation. The size-match of tongue length and flower size affects both male (pollen removal) and female reproductive success (pollen receipt and seed set).

I’ve noticed a small goldenrod on the East Glacier trail near the cliff that sports purple mountain saxifrage in spring and offers a lookout toward what’s left of the glacier. Called northern goldenrod (Solidago multiradiata), it tends to favor rather dry areas in meadows, on rocky ridges, or gravel bars. The yellow flower heads occur in more or less flattened clusters. This plant is much shorter than the Canada goldenrod, which likes disturbed areas and bears its many flowers in large, tapered inflorescences. The small flowers of goldenrods are visited by butterflies, bees, and many other insects, but which ones are the good pollinators and which are just thieves?

Near that same cliff, I saw several common harebells (a.k.a. bluebells of Scotland; Campanula rotundifolia). Found in open areas, rocky or grassy, this perennial is seen in many places around Juneau. The flowers are purple-to-blue, borne singly on each branch, but some plants have several branches on their wiry stems and may have several flowers.

Photo by Bob Armstrong

Common harebell has a broad geographic range over Europe, where it originated, and North America. It survived the advances of the glaciers, which temporarily isolated populations in different areas. Harebells in many of these populations became polyploid, having two or more complete set of chromosomes, which is likely to affect many floral traits, perhaps in different ways in different populations (as found for other species), but this question has not been investigated for this species (as far as I have found). Despite its species’ name (rotundifolia), the round basal leaves disappear early, often before flowering, and the stem leaves are not round.

Harebells are pollinated mainly by bees. The flower is protandrous, meaning that when the flowers first open, they are male, with pollen ready to disperse. Later, when the receptive stigma is mature, the flowers are mostly female. The flowers are self-compatible (as found in experiments), at least in some populations, but self-pollination results in fewer seeds than cross-pollination; in the wild, protandry prevents most self-pollination. Bees collect only pollen from male-phase flowers, but they collect both pollen and nectar from female-phase flowers. Flower size can vary from place to place, and so would the size of the main pollinating insects.

Common harebells (and probably other harebells too) form mycorrhizal associations with several species of fungi. One study found that this association had no effect on seed size or number but led (unexpectedly) to decreased growth and flower production. However, the seedlings of mycorrhizal harebells grew better than those from parents that were experimentally prevented from having that association. So the advantage of the fungal connection appeared in the next generation. Interesting!

Perceptive readers may well generate lots of follow-up questions from these brief notes!

Making a flower

Animal-pollinated plants typically make a flower to attract visiting animals. A straight-forward example is a salmonberry flower, familiar to all of us: there’s a circlet of pink petals surrounding a yellowish center composes of male (stamens) and female (pistils) parts. In other cases, Mother Nature has designed a more complex arrangement of colorful petals, as in lupines or orchids.

Turn the salmonberry flower over and look at its underside, where it attaches to the stem. There is a more-or-less cup-like structure (the calyx), composed of several elements called sepals, which are usually green. The calyx covered the developing flower bud and often remains behind an open flower, providing support.

In some species, however, the sepals have become part of the flower—they are colorful and not hidden behind the petals. The fireweeds are good examples: the four wide, pink petals alternate with narrow, darker pink sepals. Three-leaf goldthread, common in subalpine meadows, has several white sepals surrounding stamens, pistils, and some golden petals, shaped like tiny trumpets, that are serving as nectaries.

Fireweed flowers have narrow, dark pink sepals between the wide, paler-pink petals, possibly making an added attraction. Photo by Kerry Howard

Red columbine is a more complex example. Here, the petals are the five yellow funnel-like structures that have long, reddish nectar spurs projecting from the other end of the flower. The five red, flaring wings are the sepals.

The view of a red columbine flower for an approaching pollinator: five spread-out red sepals and five yellow petals opening into reddish nectaries extended behind the flower. Male and female parts protrude in front of the openings and would be contacted as a pollinator probes for nectar. Photo by Kerry Howard

The wild iris goes still further—the three drooping, purple pieces are the sepals, comprising the main part of the flower. The petals are very small, just visible between the bases of the purple sepals. In domestic, horticultural irises, the petals have become much larger, standing up above the sepals (assuming that the ancestor of domestic irises was similar to our wild iris, horticulturalists presumably have selected for greater petal size over a number of generations).

Some plants have abandoned petals altogether, making a flower of showy sepals: marsh marigold, narcissus anemone, Sitka burnet. So, if you look at the back side of those flowers, you see no sepals.

However, one cannot conclude from an observation of ‘no sepals’ that the sepals are in the flower somewhere. That’s because some species drop their sepals fairly early in floral development; baneberry and some buttercups do so.

Attractive displays sometimes incorporate elements that are not, technically, part of the flower. I’ve previously mentioned dwarf dogwood with its white bracts (modified leaves) around the central flowers. Perhaps the best known plant using bracts to make a conspicuous display are the horticultural poinsettias so commonly sold for winter holidays. The sometimes rather colorful but very small flowers are clustered in the middle of all those gaudy bracts.

Do all those details of botanical anatomy matter? Not to those for whom flowers are just part of the scenery. At a trivial level, attention to such details as a bit like a game, a jigsaw puzzle, accounting for all the pieces and fitting them together. At another level, the various contrivances that make a ‘flower’ suggest questions about the relationships with pollinators (and other possible factors) and thus (eventually) to understanding more about the lives of these plants. Here are a few such questions (and no answers, although they could be addressed by experiments or, in some cases, by close examination of the evolutionary history):

–Are fireweed flowers with colored sepals between the petals more attractive to pollinators than those that lack them?

–Colored bracts or sepals around a flower can make the display larger. How does that affect pollinator behavior?

–Horticultural irises, with their tall petals, are (presumably) more attractive to humans than the wild forms with the tiny petals. Although gardeners may not care, do the pollinating insects such as bees react differently?

–Does converting showy petals to nectaries (albeit colorful ones), as in three-leaf goldthread, allow more nectar to be produced or favor visits by certain kinds of insects?

–In the case of columbines, the elongated petals make a nectar spur that can only be accessed properly by long-billed birds or long-tongued insects, effecting pollination. Many other species in the buttercup family have showy petals bearing small nectaries. The nectary development and specialization in columbines included making the petals less conspicuous, except when the sepals are fully reflexed or when viewed from below the pendant flower. Did that development make it useful for the sepals to be so showy? 

–And why do some plants just drop their sepals, while others have no petals but only sepals in the ‘flower’.

As usual, asking questions leads to still more questions. But if curious naturalists didn’t  keep asking, there are many things we’d never know.

Hawk moths

Bedstraw hawk moth (Hyles gallii) Photo by Bob Armstrong

If you walk through a field of fireweed, you might see spittle bugs and aphids and –if you are lucky—a hawk moth. They hover at flowers on their rather narrow wings, extending their ‘tongues’ to extract nectar. They can fly very fast, which may have given them the common name of ‘hawk.’ They’re also called sphinx moths: when a caterpillar is at rest, it raises the front part of its body and tucks the head down; someone with a vivid imagination saw a resemblance to the famous Egyptian Sphinx.

There are well over fourteen hundred species of hawk moth in the world. Some of them specialize on extracting nectar from orchids and often incidentally (to the moth) pollinating them. Many hawk moths have long proboscides (‘tongues’), suitable for extracting nectar from the long nectar spurs of certain flowers. When there is a good fit between the length of the spur and the length of the proboscis, the moth can pick up pollen on its eyes or face and transfer it to another orchid flower. If the proboscis is too short, the moth can’t reach the nectar in a long spur and is not likely to visit that kind of orchid very often. If the proboscis is too long, the moth’ head or body will not contact the place in the flower where the pollen is produced, so although the moth can steal some nectar, pollination is unlikely (unless it happens that the pollen is contacted by the proboscis itself).

Certainly the most famous of these relationships (as I mentioned in an earlier essay) concerns the Madagascar star orchid, whose prodigiously long (eleven inches or so) nectar spur caused Darwin to predict the existence of a suitable moth with an equally long proboscis. Sure enough, someone else eventually found that predicted moth, in action. Now there are rumors that a second species of long-tongued hawk moth can also reach the nectar and do some pollination of this orchid.

A little closer to home, in the swamps of Florida and Cuba, the ghost orchid lives high up on tree branches. The nectar spur is said to be about five inches long (but variable) and it is now known that several species of hawk moth can pollinate this species. Unfortunately, the orchid is now quite endangered, in part because of over-collecting by too-avid horticulturalists.

Slightly closer to Alaska, in the tall-grass prairies, the fringed prairie orchids are pollinated by hawk moths. The western fringed prairie orchid has a nectar spur over two inches long, which is said to be longer than most other North American orchids. It is known to be pollinated by four species of native hawk moths (perhaps more) and by one non-native species (that was introduced to North America from Eurasia to help control an invasive weed). This species of orchid is designated as ‘threatened’, largely because of habitat loss as the prairies were plowed under for agriculture. But in addition, the moths are at risk from pesticides drifting over from the agricultural fields. Some published accounts say that no seed set is accomplished in the absence of moth pollination, but others say that a little self-pollination without the help of the moths is possible. In either case, reproduction is generally poor.

In Alaska, little seems to be known about the relationships of hawk moths to flower pollination. Of the seven species on record in the state museum, only some are represented by more than a few specimens (thanks to the helpful entomologist, Derek Sikes, for this info!). I’ll summarize a bit about three of them. Here in Juneau, we sometimes see the bedstraw hawk moth (Hyles gallii) as the adults visit fireweed and other flowers. Two local orchid-watchers have seen and photographed this moth visiting the white bog orchid, carrying pollen on its fairly long proboscis (about an inch long). Is this moth a regular pollinator of this orchid? Hawk moths elsewhere are known to pollinate related species of orchid, but it has been thought that this orchid is pollinated mostly by moths that sit on the flower while they sip nectar (instead of hovering, as a hawk moth does). In any case, hawk moths seem to be uncommon around here, so their role in pollination may be occasional at best.

Another hawk moth that we occasionally see here is called the hummingbird hawk moth (Hemaris thysbe). The wings are clear, without any colorful scales. This species is known to visit many kinds of flowers, including some orchids similar to the white bog orchid, but how many of these visits accomplish pollination is not known. The proboscis is of medium length (less than an inch), so deep nectar sources are not available to this moth.

One other fairly well-represented species in the museum collection is the one-eyed sphinx moth (Smerinthus cerisyi), which sports pretty blue eyespots on the hind wings. Although I don’t know if it has been recorded in Juneau, there are records from coastal British Columbia and from near the head of Lynn Canal, so it seems possible that we might see it here. It has an extremely short proboscis (only a few millimeters long), and one source states that it is not functional at all. In that case, the adults would not feed and there would no pollination.

Hawkmoth caterpillars are often called horn worms, for the horn-like projection that sticks up from the rear end. The three Alaska species that I’ve mentioned all have ‘horns’, although not all hawk moth caterpillars do. The caterpillars are herbivorous, commonly eating a variety of leaves. Bedstraw caterpillars eat fireweed, plantain, enchanters nightshade, and many other things, in addition to bedstraw. Hummingbird caterpillars eat snowberry, blueberry, cherry, thistle, clover, and more. One-eyed sphinx caterpillars forage chiefly on willow and poplar, but occasionally other species too.

Late summer flowers

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 resources.to 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.

Potluck

You never know what might be offered at a potluck supper, where you browse over a miscellaneous collection of dishes. This essay is a bit like that—an assortment of unrelated but potentially interesting observations and information.

Last week I mentioned the great abundance of mosquitoes in June in the Interior. Mosquitoes can surely be a major nuisance and in some regions of the world they carry diseases and parasites. But is there another side to this coin?? What good are mosquitoes?

Mosquitos are good bird food. Swallows and swifts catch them on the wing. Certain warblers and flycatchers dart out from a perch to catch them as they fly by. I’d bet that the female hummingbird swooping back and forth in my front yard catches some, too. Mosquito larvae are aquatic and provide good prey for small fish and for larger invertebrates that are also prey for fish. Dragonflies and damselflies feast on them.

Mosquitoes are more than unwilling prey, however; they are the principal pollinators of a diminutive-flowered plant called the small bog orchid. In Alaska, males and females of several species of mosquito are known to visit these orchid flowers and carry pollen from on flower to another (other insect visitors include small moths and flies, but their role in pollination is undocumented). As the mosquitoes poke into a flower, globs of pollen are slapped onto their eyes, where they stick until the mosquito goes to another flower, which is arranged in a way to pull off the pollen, leading to seed development. There may well be other kinds of small flowers that are pollinated by mosquitoes: male mosquitoes commonly live on nectar from flowers and even the blood-sucking females do to in some cases. Some of these visitations might achieve pollen transfer, but sometimes the mosquito might be just a nectar thief.

Flower color is one of the cues used by flower-visiting animals. Color helps identify the species, sometimes the age of the flower, and in some cases, whether or not it has already been visited by a pollinator. Usually, all the flowers of a particular species are the same color, or nearly so. But we see exceptions. For example, almost all of the chocolate lilies we see have brownish flowers, but very rarely we see one with yellowish flowers. Wild lupines normally have blue to purple flowers, but in rare cases we find one with pink flowers. Occasionally, we’ve found white-flowered individuals of fireweed, northern geranium, shooting star, and bluebells (whose flowers are normally pink or blue). These very unusual flower colors are presumably the result of genetic mutations.

Photo by Denise Carroll

These mutants don’t seem to spread and become more common in their respective populations. But why? Do pollinators discriminate against those oddballs, leaving them unvisited, unpollinated, and without offspring? Or, maybe the (presumed) gene that controls flower color also controls something else in the plant’s life, something that interferes with some aspect of normal function. Multiple effects of single genes are common. As usual, a simple observation leads to more questions.

Bumble bees are very important pollinators of many kinds of flowers. They are the principal pollinators of monkshood, lupine, blueberries, iris, louseworts, and beach pea, which have flowers that must be manipulated in a certain way in order to achieve pollination. But the bees also join with a variety of other critters to pollinate roses, salmonberry, thimbleberry, goldenrod, and columbine (to mention just a few).

However, all across North America, from California to the east coast, populations of some bumblebee species have become very rare or even disappeared entirely. Because I have a very unscientific impression that I see fewer bumblebees on our wildflowers in the last two years than previously, I wondered if our bees may also have crashed. I asked the UAF expert, who said that a study currently underway in Denali is designed to detect major changes in bumblebee populations there, but as so often happens, these seem to be no scientific data for Southeast. We can hope that our bumblebees are in good shape, because so many of our wildflowers depend on them.

Finally, and just for fun (?dessert at a potluck?): One day I walked through the Eagle Beach day-use area after a stroll on the beach. I spotted a big black bird sitting on a rock and holding a chunk of something red and drippy in its bill. Through binoculars, I saw that the raven held a succulent piece of watermelon. Just beyond the rock was a picnic table with several plastic containers, temporarily abandoned by a human family that was playing out on the intertidal sand flats. At least two of those containers had the lids removed (?by the raven?) and one of them still held a chunk of melon. That family was in for a surprise when they came back to their table!

Purple mountain saxifrage

One of the earliest flowers to appear in spring is purple mountain saxifrage. In April some of us make a point of regularly checking certain places where we know it lives, just for the pleasure of watching for the first open flower and then the appearance of more and more blossoms, until there are multiple patches of the pinkish-purple flowers on some of the local rocky outcrops.

Photo by Bob Armstrong

This low-growing plant occurs in Arctic regions around the world, and in alpine areas of central Asia, Europe, and North America. It’s a tough little plant, quite resistant to drought and water-stress. It forms associations with mycorrhizal fungi that provide nutrients and water, in exchange for carbohydrates produced by the saxifrage leaves. As with several other early-season bloomers, the flower buds are actually formed the year before the flower opens, but eggs (in the ovary) and sperm (in pollen) don’t develop until spring.

Female parts of the flower mature before the male parts do, which reduces the chance of self-pollination (pollen fertilizing future seeds in the same flower). Most seeds are produced by out-crossing (pollen fertilizing future seeds on a different plant).

The flowers are pollinated by insects of various sorts, including bees and flies. Early in May, we watched a female margined-white butterfly visiting one flower after another, so presumably there are minute amounts of nectar therein. These insects don’t see the longer (reddish) wavelengths, so they see the flowers as bluish. However, studies have shown that seed production is often limited by low levels of pollinator activity, perhaps in part because bad spring weather sometimes reduces insect activity. In addition, one study showed that as soon as other flowers started to bloom, insect visitation to the saxifrage decreased, as the insects found preferred sources of food.

This is an ecologically variable species, with different types adapted to different conditions of soil, snow-melt, length of growing season, and so on. For example, one study showed that the plants growing in cold, wet soils with late snow-melt had higher metabolic rates and faster production of shoots than those in warmer, drier sites, but they did less well at storing carbohydrates or water for hard times. In some areas, there are two growth forms that grow side by side but differ in structure (prostrate vs cushion-like) and in reproduction: one does better at seed production but the other excels at propagating by shoot fragmentation.

On the Old World Arctic tundra, purple mountain saxifrage flowers and old seed heads are eaten by barnacle geese when they arrive on the nesting grounds, and reindeer eat it too. I have not found information on animals that consume this plant in North America.

A word of caution: If you see this pretty plant in the wild, please do not remove it! That deprives lots of other folks of the pleasure of finding and seeing it in its natural setting.

Flowery fun in Gustavus

Lady’s slipper orchids are sometimes called moccasin flowers, referring to the shoe-like shape of the flower. One of the petals is modified to form an oval pouch with an opening on top. The edges of the pouch are rolled inward. A small shield-like structure hangs down into the back of the pouch and behind the shield are the sex organs. The flower offers no nectar to visitors, but at least some species have an attractive aroma.

Bees that visit these flowers enter the pouch, but the rolled-in edges keep them from crawling out. So, once in the pouch, the bees are obliged to crawl up behind the shield, in order to get out again. In doing so, they pass very close to the pollen-receiving stigma, leaving pollen from previously visited flowers, and the pollen-bearing stamens, picking up pollen on their bodies to carry to another flower. A very elaborate system for creating the next generation of lady’s slippers.

After pollination, thousands of dust-like seeds are produced. They are so small that they contain no nutrition for a developing embryo (this is true of orchids in general). Lacking a source of nutrition, the seeds have to rely on forming an association with certain fungi (mycorrhizae), in order to germinate and grow. Lady’s slippers are slow growing and take several years to reach the flowering stage.

There are dozens of species of lady’s slippers in North American and Eurasia. They belong to the genus Cypripedium. This name is derived from some ancient Greek words. Cypris is an old name for Aphrodite (a.k.a. Venus in Latin), the goddess of love and beauty. The ‘ped’ part of the name refers to foot or footwear, sometimes rendered as ‘sandal’. So Cypris/Aphrodite/Venus has a rather large collection of sandals in her wardrobe!

Lady’s slippers were familiar to me, from years spent in the Midwest, but I have never seen them in Juneau. So one of my hopes for a recent Gustavus trip was seeing these in bloom. We’d seen their leaves occasionally in the past, but the plants were not then in flower. On this June trip, with the help of a knowledgeable naturalist there, we located clusters of three species of Cypripedium. There was a large-flowered white one (C. montanum, or mountain lady’s slipper). A small-flowered, round white one with some brownish spots is called C. passerinum (sparrrow’s egg or northern lady’s slipper). A yellow-flowered species has often been classified as a subspecies of C. calceolus, but more recently botanists seem to consider it to be a separate species, C. parviflorum, the small yellow lady’s slipper.

Cypripedium passerinum, sparrow’s egg lady’s slipper. Photo by Kerry Howard

Lady’s slippers and many other showy orchids are often collected from the wild by willful gardeners. But this practice has led to the near-extinction of some species. The slow-growing habit, low levels of pollination and seed set, and the need for mycorrhizal fungi make recovery of exploited populations slow and difficult. So these plants should never be harvested from their native habitats.

Cypripedium parviflorum, small yellow lady’s slipper. Photo by Kerry Howard

We found other orchids too. Tiny twayblades are much more common in Gustavus than in Juneau. They are pollinated by minute flies and wasps, as Darwin documented long ago. Coralroots and so-called rattlesnake plantain are common in Juneau as well as Gustavus.

Orchids were not the only flower show in town, however. Lupines created hills of blue on the beach dunes. Cow parsnips and buttercups brightened beachside meadows. Roses and irises added splashes of color. One meadow was thoroughly decorated with the small white inflorescences of Tofieldia, which is easier to say than the ponderous common name of sticky false asphodel. Sticky it is—the stem sometimes captures tiny insects. Apparently, some botanists thought the inflorescence resembled the European asphodel, which in Greek mythology grew in the meadows where the souls of the dead walked. Great stretches of forest understory were carpeted by the leaves of deerheart, which sent up its small white spires of flowers, and the nearly-luminous, wide, white flowers of bunchberry (one of my companions is alleged to have said that they lighted the way to the outhouse in the darker hours!).

Indian paintbrush provided the most stunning floral array. Here in Juneau we see some yellow-flowered ones and (especially at higher elevations, I think) a few red-flowered ones. But in Bartlett Cove we found a beach meadow simply covered with paintbrush flowers: yellow, red, orange, particolored, and every combination in between. Quite splendid.

Pollination tricks

Flowers are a plant’s way of sexual advertisement. They are evolutionarily designed to attract animals that visit the flower in hopes of collecting nectar or pollen to eat, inadvertently accomplishing pollination. Insects are the most common type of animal that provide this service for flowers, although some insect visitors are thieves, taking the food but without pollinating.

To accomplish pollination, the foraging insect, as it rummages around inside a flower, incidentally gets pollen on its head or body or legs from the male parts (called anthers) of a flower and accidentally brushes off the acquired pollen on a receptive surface (called a stigma) of the female part of a flower. Many flowers have the means of avoiding self-pollination (within the same flower or between flowers on the same plant) and promoting cross-pollination from another plant, which creates greater genetic variation among offspring, one of the chief advantages of sexual reproduction. But that’s another, long story, so here I’m focused just on some behavioral interactions of insects and flowers.

Flowers have evolved many ways of controlling the visits of their pollinators, so the insects enter the flower in a particular way that effectively removes pollen from the anthers and deposits pollen on a stigma. The variety of ways in which plants do this could be the subject of several books; indeed, Darwin wrote one just about The Various Contrivances by which Orchids are Fertilised by Insects.

I’m not about to write a whole book here, so I’ll just describe a few ways some of our local flowering plants accomplish pollination. Open, saucer-shaped flowers are available to most insect visitors. A rose, for example, produces anthers and stigmas right in the middle of a circlet of petals, and all an insect has to do is walk around in the middle of the flower, sipping nectar and casually picking up or depositing pollen when it happens to contact the anthers. Nothing to it! Almost any bug can do it.

Much more interesting and intricate mechanisms of pollination exist in our local flora. For example, twayblade orchids grow profusely in young conifer forests, such as in Gustavus or near Eagle Glacier cabin. The flowers are tiny, pollinated by very small bees and flies. When the insect seeks nectar, it triggers the explosion of a drop of sticky stuff that picks up pollen on its way out of the flower and sticks the pollen to the head or eye of the insect, where it is cemented. Some parts of the flower actually move apart in order to make space for the exploding drop and pollen to emerge and fasten to the insect. Later, when the insect visits another twayblade, the pollen is detached somehow and deposited on the stigma, sometimes leaving the congealed sticky blob behind on the insect. Darwin spent a lot of effort figuring this one out!

A very different pollination mechanism is used by lupines, which are pollinated by bees. The sexual parts are hidden away in a fold between two fused petals in the lowest part of the flower. A bee pries open the fold as it probes for nectar. When it does so, out pop the sexual parts, and pollen can be dusted on the bee or brushed off the bee onto the stigma. When the flower has been visited, the uppermost petal has turned from white to pinkish-purple. The color change is a signal to future bee visitors that the nectar is depleted and the bee should visit other flowers on the stem.

Louseworts, in contrast, hide the sexual parts in a little hood in the upper part of the flower. So a visiting bee has to get into the flower in a certain way, often wedging open the hood, in order for its body to contact anther or stigma. One of the louseworts in Southeast, however, does it differently, as described the next paragraph.

“Buzz pollination’ is one of the most interesting and common pollination mechanisms in our area. In this process, a bee (usually) lands on the flower and buzzes in a special way. Its major wing muscles are temporarily inactivated, and its body just vibrates very rapidly, repeatedly hitting part of the flower. The vibrations shake dusty pollen onto the bee’s body, to be deposited eventually on a stigma. (Obviously, this technique doesn’t work with sticky pollen). Buzz pollination is how shooting stars, tomatoes, blueberries, wintergreens, and many other flowers get pollinated. Buzzing may also contribute to pollination even in open, saucer-shaped flowers such as salmonberry and roses.

Bog laurel. Photo by Bob Armstrong

Bog laurels grace our muskegs with their pink, wide-open flowers. If you look closely at a young flower, you would see that the female parts are in the center. But the male parts consist of arched, white filaments leading from the flower center to small, dark blobs (the anthers) that are nestled in pockets on the surface of the petals. When a bee lands on the flower, the spring-loaded filaments straighten, raising the anthers, and shaking pollen on the buzzing bee. So you can tell if a bee has been there by looking at the position of the filaments (arched or straight) and seeing if dark anthers remain in the petal pockets.

 

I’m sure there are other cute tricks by which our local plants contrive to deliver pollen from flower to flower. See if you can find some!

Tricky flowers

Most of our wild flowers are wide-open structures, just letting all the sexual parts hang out. Think of nagoonberry or cloudberry, asters, avens, silverweed, wild roses, geranium, anemones, and so on—all of these just present the sexual organs to whatever insect happens to land there. The smaller flowers of angelica and cow parsnip and their relatives do the same, but present the flowers in flat-topped bunches, making a good-sized landing platform for a visiting insect. It is then a relatively simple matter for an insect to walk around, picking up pollen from one flower and carrying it to another. Columbine and fernleaf goldthread dangle the sex organs loosely, downward or outward, where a visitor just bumps into them, when in search of nectar deeper in the flower.

Some of our flowers, however, are a bit more complex, requiring a visiting insect to do a little work or behave in a particular way. In these species, the sexual parts are typically enclosed within the flower—concealed in various ways. Here are some examples:

Lupine. Photo by Bob Armstrong

Lupine: Bees pry open the flower, and when they depress the lowest, keel-like petal, out pop the stigma (to receive pollen, if the bee carried any) and the anthers (containing pollen to be deposited on the bee and carried to another flower).

Twayblade orchid: A visiting tiny bee or fly pokes into the miniscule flower, bumping into a projection that releases a sticky gob that pulls out clumps of pollen. The pollen is stuck onto the insect’s face or head until another flower is visited and the pollen is inserted there.

Violet: Down in the heart of the flower, the stigma is encircled by closed anthers, packed tightly together (the technical term is ‘connivent’—conjuring up mental images of conniving and scheming deviously (!). A visiting insect displaces the stigma, pushing it to one side and perhaps depositing pollen, and only then do the anthers open, releasing pollen to be picked up and carried away by the insect.

Bog cranberry. Photo by Bob Armstrong

Blueberry, cranberry, shooting star, wintergreens (and tomato): Although the flowers differ in shape, all depend on what is called ‘buzz pollination.’ A visiting bee vibrates certain flight muscles (and buzzes), which causes pollen to shake down on the bee. If the bee already had pollen on its body, from another flower, it is brushed off onto the stigma.

Bunchberry/dwarf dogwood: the tiny flowers are clustered together, surrounded by white, petal-like bracts. Ripe flower buds open suddenly and the anthers explode their pollen into the air or onto an insect, when a tiny projection on one of the four petals is triggered, perhaps by an insect.

Lady-slipper orchid/moccasin flower: These flowers are doubly devious. They offer no nectar to insect visitors, who nevertheless prospect around inside the ‘slipper’, in hopes of a reward. But once inside that slipper, they cannot get out—except by squeezing through a tight opening at the back of the flower, where the sexual organs just happen to be located, convenient for pollen deposition and pick-up.

Bog laurel. Photo by Bob Armstrong

Bog laurel: When the flower opens, the anthers are held in little pockets on the faces of the petals, with slim filaments linking them to the center of the flower. This species is normally pollinated by bumblebees: when the bee lands on a flower, the anthers spring out of their pockets and dust pollen on the bee. The springing mechanism is reported to be very sensitive, so perhaps even small insects, coming in search of nectar, can spring the anthers free, but it is unclear if the pollen would land on their bodies and if they would be effective pollinators.

All these clever little arrangements are a small sample of the ingenious contrivances for pollination exhibited by flowers in more southerly latitudes, about which whole books have been written. The world of flowers is far more complicated than one might expect.