Owen Sholes

Flowering plants produce seeds through the process of pollination, the release of pollen (containing sperm cells) from stamens, and the transfer of pollen to the receptive surface (stigma) of the pistil (containing egg cells).  

Flowers

Goldenrod flowers (see my earlier posts) are clustered in flower heads.  The flowers on the edge (ray flowers) have large petals and are solely female (no stamens).  When a flower head opens, the first flowers to bloom are the ray flowers, which (presumably) provide nectar as a reward for any pollinating insect, and which can receive pollen.

The disk flowers in the middle of the head are both male and female.  When the head first opens, the disk flowers are closed (see their blunt, closed ends in the photo above).

But later, the disk flowers open and the stamens thrust the anthers upward, exposing the pollen above the surface of the flower head (see photo below).

Insects

Many insects visit goldenrod flowers.  I posted a one-minute video (Twitter and Facebook) of pollinator frenzy on a couple of goldenrod inflorescences.  The video is full of wasps and bees, and goldenrods also attract beetles, ants, flies, butterflies, moths, and more.  These insects get nectar and/or pollen as a reward (and some probably nibble on the tissues of the flowers).  For some, the reward is immediate and short-term, fueling their activity during that day.  For others, the reward is long-term, as they take the pollen and/or nectar back to their nest to share (for social insects) or to provision the nest for their young when they hatch.  

Pollen is a durable, nutrient-rich food.  Nectar is almost entirely sugar and water, an energy drink.  A few species can evaporate the water to produce honey, a sugar solution so highly concentrated that it won’t spoil.   I posted about honey earlier.

Pollination

As the insects cling to and walk on the flowers, they are almost certain to pick up some sticky pollen on their bodies.  They might ignore the pollen, or try to clean it off, or eat some of it, or gather some of it to take with them, but no matter how carefully they groom themselves, some pollen grains are likely to be scattered on their bodies.  When they move on to other flowers, some of those grains will rub off on stigmas, the topmost part of the female portion of a flower.  If the pollen from plant A rubs off on a stigma of plant A, that will be a dead end.  Goldenrods are self-incompatible (they can’t fertilize their own flowers).

But pollinators on goldenrods are greedy, abundant, and highly mobile.  They will bump into each other, knock each other off of the flowers, and cause each other to fly to other flowers, some of which will be on another plant.  Or they will drink the flowers dry and fly a bit to find more flowers, sometimes on another plant.

If the pollen from plant A ends up on the stigmas of plant B, then fertilization can get going.

Fertilization

The process of fertilization in flowering plants has several steps, and some of them seem at least somewhat improbable.  But all flowering plants do it, and there are thousands of species of flowering plants.  As complex as the process seems, they make it work.

When a pollen grain contacts the stigma of a flower, the grain “germinates.”  A tube, called the pollen tube, grows from the pollen grain, and two sperm cells, both haploid, venture down the tube.

The stigma is the topmost portion of the pistil, the female portion of a flower, and the tube penetrates the stigma, the style below the stigma, and enters the ovary below the stigma.  Inside the ovaries there are ovules, each surrounded by layers of cells, and each containing an embryo sac, a small structure with eight cells or nuclei, all of which are haploid.

The pollen tube finds its way into the ovary and then into an ovule and embryo sac.  One of the sperm cells will fuse with one of the female haploid cells, the egg cell, and that fusion produces a diploid zygote, the first cell of a new plant.  The zygote begins to divide and produces an embryo, which will stop growing and become dormant within the ovule.  The ovule will eventually become a seed.  

The other sperm cell inside the embryo sac fuses with two of the female haploid nuclei to produce a triploid cell (I warned you that the process was weird), and this triploid cell divides by mitosis multiple times to produce endosperm within the seed.  Some plants have almost no endosperm (orchids), and others have a lot (coconuts).  Goldenrods have enough to serve as a nutrition source for the seed when it germinates.  Goldenrod seeds are small and light, easily blown around to places at some distance from the parent plant.  But they also have a bit of endosperm to help the new plant to grow, wherever it lands.

Each goldenrod flower has one ovary with one ovule, so each flower produces, at most, one seed.  The ovary provides a covering around the seed, which is technically a fruit, a dry fruit called an achene.  As the ovary wall dries out, small filaments remain attached to the top of each seed.  These filaments (the pappus) provide wind resistance and help the seed disperse away from the parent plant sometime in the next several months.  Most of the seeds will fail for one reason or another, but there is always a chance that some will find a place to grow once the next spring rolls around.

Sexual reproduction

This whole sequence of events is sexual reproduction: flowers attract pollinators, pollen transfer leads to fertilization, fertilization leads to seeds and dry fruits, and dry fruits blow around in the wind.  Each offspring has half of its genes from one parent and half of its genes from the other parent, and thus is genetically different from both them.  Each unique seed then has a chance to colonize new habitats.  It’s a risky business, but it has worked for an unfathomably long time.

If you feel lost in the details, take a look at the video on Twitter or Facebook.  The frenetic pollinators can’t get enough of the goldenrod flowers.  All that messing around leads to pollination, an essential part of producing another generation of goldenrods.  And there it is, right by your house, along the road, or in whatever habitat your local goldenrods occupy.  It won’t last forever, so take a look while it’s going on.  If you miss it, it will all happen again next year.  Nature is persistent.

Sometimes, a name tells you a lot.  Shining flower beetles are shiny, they can be highly abundant on flowers, and they are, indeed, beetles.  Some of their relatives are called shining mold beetles because they are abundant on fungi.  All are in the family Phalacridae.

These beetles are also small, which probably makes the mold beetles easy to overlook, but the flower beetles are right there on top of the flowers, really hard to miss (if you stop to look).  They are especially shiny in sunlight, and their brown color contrasts with the bright color of the flowers.  They appear to be bite-sized for a bird, but I have never seen a bird grab one (though it would be so quick that the chances of seeing it are tiny, just like the beetles).

In the cool of the morning, the beetles are mostly between the flower heads, an indication that they spent the night there.  In the middle of the day, they are often on top of the flowers, heads down, giving every indication that they are finding something on or in the flowers to eat.  Their mouthparts probably cannot reach nectar, but pollen and tender floral tissue are right where their mouths are.

Grass-leaved Goldenrod (a necessary digression)

Decades ago, grass-leaved goldenrod was classified in the genus Solidago (S. graminifolia), but taxonomists decided that it, and similar species, should be in a separate genus, Euthamia.  When it blooms, the flowers are yellow and look a whole lot like those of any other goldenrod around them.  

So?

As you can see in the picture, shining flower beetles congregate on E. graminifolia flowers as soon as the first ones bloom.  They spend little time on unopened flower buds, but can’t seem to get enough of the yellow blossoms.  As far as the beetles are concerned, a goldenrod blossom is a goldenrod blossom, no matter how the plant is classified.

Where do shining flower beetles spend the winter?  Good question.  I would guess that the females lay eggs somewhere on, in, or near the plant, where the eggs remain dormant until the spring.  The larvae live among flowers, but I don’t know whether the eggs wait to hatch until goldenrods are available.  I suspect not, and I suspect that they have more than one generation a year.  It would be interesting to find out.

Goldenrod plants are getting tall now, getting flowers ready to bloom, and becoming food for a variety of insects.  One of the herbivores is conspicuous, if you take a little time to notice.  Some stem tips look vigorous with leaves facing upward and outward and flower buds developing, like the picture above, but other stem tips are closed up and bent over, with the leaves facing inward and down.  The outer surfaces are still green and, at a tissue level, seem healthy.  But something is wrong.

Many plants show deformities, so these clusters of leaves might be some kind of developmental problem or pathogen infection.  But no.  If you pull the leaves apart carefully, there is white material holding the leaves together, and at the very center, there is a tiny moth larva.  The white material is silk, spun by the caterpillar – yes, even one so small – tying the leaves together, holding them close to the caterpillar for protection and for food.

When I looked up “leaf tier” for goldenrods (pronounced “tie-er,” not “teer”), several species came up, representing multiple families, such as Tortricidae and Gelechiidae.  I don’t have access to a microscope, so I can’t be sure which species I’m finding (and even with a microscope, species identification would be quite difficult), but they are doing well, whatever they are.

Most of the stem tips are undamaged, soldiering onward and upward.  But they are beginning to show their age as insects find places on them and in them to live and to eat.  The food web and growing season are marching forward together.

I already talked a bit about goldenrod hybrids and chromosomes.  I will talk later about goldenrod reproductions, but briefly, they produce seeds that blow around (sexual reproduction) and rhizomes that spread underground (asexual reproduction).  As a result of mutation, sexual reproduction, and hybridization, there is considerable genetic variation within any one species of goldenrod, and within any one population of a single species.

The photos at the top of this post are an example (all three are Solidago juncea, early goldenrod, July 30, 2021).  They are growing on the same side of the same road, all within the same five-meter stretch of roadside.  One is almost done blooming, one is in the middle of blooming, and one has yet to bloom.  Yes, there could be some environmental differences between the locations of these plants, but it is highly unlikely that such minor differences would account from the considerable differences in phenology.  No, the differences are mostly genetic, and are representative of the amount of genetic variation in goldenrods.

So what?

As it turns out, genetic variation has a measureable effect on what goes on in a field of goldenrods.  Among the early goldenrod plants along my road, the differences might make it impossible for the earliest and the latest blooming plants to cross pollinate.

But there is more.

Gregory Crutsinger and his colleagues examined the significance of genetic diversity in an excellent field experiment published in 2006.  First, they had to find a variety of goldenrods and clone them (they used tall goldenrod, S. altissima).  Goldenrods are easy to clone because they spread vegetatively, but it takes time to grow enough clones.  This experiment required long-term planning.  And they didn’t merely assume that the different plants were genetically different.  They compared the DNA of the different clones using a method called amplified fragment length polymorphism (AFLP).  As expected, and confirmed, there were differences in the DNA between clones.  

They planted the goldenrods in plots at a density of stems equivalent to those in many goldenrod fields in northeastern North America (12 stems per square meter).  All plots had the same number of stems, but some plots had only one genotype, while others had three, or six, or twelve genotypes.  They replicated each type of plot many times, and scattered each type of replicate throughout their test field to avoid bias by location.  In short, they did it right.

They identified the arthropods (insects and arachnids) on the plants during the growing season, and measured the growth of the plants.

So what happened?  The graphs show us what happened.

On average (the horizontal black bars among the circles), the greater the genetic diversity of the plants, the more species of herbivores, the more species of predators, and the more species of all arthropods were found in the plots (richness means the number of species).  And the greater the genetic diversity, the greater the total growth of the plants in the plots (they measured growth as above-ground net primary production, ANPP).  

The graphs show a lot of variation among the plots, and that the effects were statistically significant.  Increased genetic diversity resulted in 20-35% increases in the average values of various measurements, which I would say is functionally significant.  

I would not have predicted this much of an effect of the genetic differences among goldenrods of the same species, so I am delighted that someone else took the time to test it.

And I think it is highly likely that genetic variation within other plant species is also significant.  Back in the 1970s, Janus Antonovics cloned a bunch of grasses that he collected in the wild, and checked to see whether the clones grew differently, whether alone or in competition with other clones.  They did. 

So genetic variation matters within a species.  

When we talk about biodiversity (how many species in a habitat, region, or the planet), we need to include genetic diversity.  Genetic diversity is the foundation for all diversity.

Reference

Gregory M. Crutsinger et al 2006.  Plant genotypic diversity predicts community structure and governs an ecosystem process.  Science 313: 966-968

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.715.81&rep=rep1&type=pdf

Postscript

You might have noticed the green sphere-like structures among some of the flowers in the pictures.  Those are galls were produced by a species of gall midge (tiny flies).  I hope to explore those in a future post.  

Growing up in northern Illinois, I would sometimes walk or run through fields of weeds, fields where many, most, or all of the weeds were goldenrods.  After some of those forays, I noticed a few blue-black insect larvae on my jeans.  I had no idea what they were, and it was immediately obvious that they were just accidental passengers, easily removed and nothing for me to worry about.

I later learned that they were the larvae of Trirhabda beetles, a genus of leaf beetles (Chrysomelidae) whose species each feed on a small range of host plants.  At least two species, T. canadensis and T. virgata, specialize on goldenrods, both as larvae and as adults.  

The beetles cling to the surface of the plant and chew up the leaf tissue.  Each beetle can eat a fair amount of leaf material in its lifetime, and sometimes, there are a large number of beetles, each roughly a centimeter long.  After most of season of feeding on goldenrods, sometimes there isn’t much leaf tissue remaining (see photo above).

One might think that these beetles, exposed on the plant, would be easy pickings for predators, and some predators and parasites do attack them.  But the beetles smell like goldenrods, a distinctive odor that results from an array of chemicals in the plant tissues, chemicals other than the content that provides nutrition for the beetles.  When the late Robert Whittaker was a professor at Cornell, he tried to feed a pet lizard some Trirhabda beetles.  Though the lizard was quick to devour mealworm larvae, it wanted nothing to do with the goldenrod beetles.  Their chemical content, taken from their goldenrod food, seemed to provide some degree of defense (more on this in another post).

I don’t have a picture of Trirhabda beetles, but you can easily find some on line (for example: https://en.wikipedia.org/wiki/Trirhabda_canadensis).

Ann Herzig performed some elegant experiments with Trirhabda beetles and goldenrods in and around Ithaca, New York.  She planted goldenrods on the roofs of some buildings on the Cornell campus because the buildings were a long way from the fields of goldernrods around the city.  In various plots, she produced different amounts of beetle-like damage to see whether the beetles responded to the leaf damage they encountered.  The beetles found these plants (clearly, their dispersal abilities were considerable) and they were more likely to stay on plants with low levels of damage.  At least some of the females that arrived on her test plots had already mated, and they laid more eggs on plots with low damage than on plots with high damage.

When she set up other plots around Ithaca with different levels of damage, beetles tended to leave plots with high damage, and they tended to stay on plots with low damage.  If she put a dab of paint on the back of the beetle, gluing the forewings (elytra) together, they couldn’t fly (this was a brilliant experimental manipulation).  The painted beetles stuck around longer than other beetles, which might seem like an obvious result, but she couldn’t be sure of their main mode of dispersal until she prevented them from flying.  

So she showed (among other things) that the beetles can find goldenrods that are far away, can detect levels of damage on the goldenrods, and will fly elsewhere if the plants are already heavily consumed.  These beetles are well adapted for the exploitation of goldenrods.

But for various reasons (most of which I don’t know), the numbers of beetles fluctuate from year to year, so the goldenrods are not always slammed by these herbivores.  The plants and insects manage to coexist over a fairly large portion of North America.  Is their coexistence the result of coevolution?  Seems like a reasonable hypothesis.

One last thought: goldenrod ball galls can reduce the weight of seeds and the number of rhizomes produced by an infested goldenrod plant.  I suspect that Trirhabda, when they are abundant, can have an even greater effect on goldenrod growth.  Seems like another reasonable hypothesis.

Further reading

Ann L. Herzig 1995 Effects of Population Density on Long-Distance Dispersal in the Goldenrod Beetle Trirhabda virgata. Ecology 76: 2044-2054

Ann L. Herzig 1996 Colonization of host patches following long-distance dispersal by a goldenrod beetle, Trirhabda virgata. Ecological Entomology 21: 344-351

Owen D.V. Sholes. 1981 Herbivory by species of Trirhabda (Coleoptera: Chrysomelidae) on Solidago altissima(Asteraceae): variation between years. Proceedings of the Entomological Society of Washington (D.C.) 83: 274–282

David C Hartnett and Warren G Abrahamson 1979 The Effects of Stem Gall Insects on Life History Patterns in Solidago canadensis. Ecology 60: 910-917

I wrote about goldenrod ball galls back in the spring when the gall flies were dormant pupae inside gray/brown galls from the previous year.  Now, adult flies have emerged from last year’s galls, mated, and laid eggs on fresh green stems.  There, the larvae have induced the growing stem tissue to swell into a gall surrounding them.  

The galls are large and conspicuous.  At least on the hillside where I found them, the galled plants are close together in a few groups, with plants in between those groups having no galls. This distribution pattern could be the result of females distributing their eggs on one plant after another as they encounter them, moving as little as possible between plants, until they run out of eggs.  Or perhaps some plants are more attractive and/or less resistant to the galls.  Because goldenrods spread vegetatively, each patch of goldenrod contains genetically identical stems.  If one of those stems resists gall flies, they all will.  If one of those stems is vulnerable to gall flies, they will all be vulnerable.  

There also is some level of interaction between plant and fly, resulting in variation in gall size.  Warren Abrahamson has explored goldenrod gall flies in detail.

The galled plants have one gall each (though sometimes there can be more).  The flies spend most of their lives inside their galls, feeding and developing from midsummer through winter and spring into the next summer.  The adult flies serve to produce another generation of galls. And the production of galls is always essential for the production of a new generation of flies.  Year after year, at least some manage to succeed.

Goldenrods are in the daisy family (Asteraceae) even though they don’t look much like daisies.  The old family name was Compositae, a clue that the flowers occur in clusters, and that what looks like a single flower (a daisy) is actually a composite (!) of several to many flowers.  The family is huge and worth getting to know because it includes a vast array of species with a vast array of characteristics.  We eat them (sunflowers), poison them (dandelions), get poisons from them (pyrethrins), use them as medicines (Echinacea), use them as décor (marigolds), curse them (ragweed), and can hardly get away from them (daisies).  

Back to the flowers.  Goldenrods produce an array of flower heads, each called a capitulum, and each containing several actual flowers.  Those flowers around the edge of the capitulum are ray flowers (with elongated petals), and those in the middle are disk flowers (with tiny petals).  In a daisy, the distinction is obvious: the ray flowers have big white petals and the disk flowers are bright yellow with hardly any petal tissue at all. 

In goldenrods, you have to look closely to see the difference.  The flowers are all the same yellow color (in most species), and the rays are not large.  But they are there.

Each flower is tiny, but the collective display – in each head, and with all the many heads combined – is substantial.

According to Gray’s Manual of Botany, the ray flowers in each head are female only (pistillate), while the disk flowers are both male and female (staminate and pistillate).  Early in blooming, the disk flowers don’t appear to be open, but the ray flowers are.  Of course, the disk flowers have essentially no petals that it’s hard to tell, at first, what “open” means.  But later, it becomes obvious (again, you have to look closely).  Then the plants provide both nectar and pollen to the insects that visit.  And the pollen gets distributed widely.  Never doubt the power of group advertising.

Early goldenrod (Solidago juncea) gets its name from blooming early.  In central Massachusetts where I live, their buds are already well formed before other species have even begun to develop their inflorescences.

Inside these buds, pollen and ovules are developing, producing haploid sperm and egg cells for fertilization.  The stamens and ovaries are providing diploid cells for support.  It’s a productive time.

The growing tissues need a supply of materials, and the phloem provides it by translocating amino acids, sugars, and more to where they can be assembled into new cells.  Some photosynthesis takes place in the buds themselves, some occurs in the green parts of the inflorescence, and some occurs in the lower leaves.  The green parts of the plant are sources of raw materials for the process of reproduction.

In southern parts of the North America, some goldenrod species bloom year round, but in northern areas, they wait until summer.  Everything happens in a small portion of the year, and some things happen so quickly that they are truly fleeting.  In this window of opportunity, seeds and their encompassing fruits (achenes) will come to be.  

The public part of the process – blooming – is not far off.  The stages are being set.  It’s going to get exciting.  At the cellular level, it already is.

A Life Cycle Tied to Early Goldenrod

You might have heard of jumping plant lice (family Psyllidae) because they pierce plant tissues with needle-like mouthparts and feed on what’s inside.  Some of them transmit diseases of plants while they feed, like citrus greening, and are therefore of great economic importance.  Others just feed.

At least one species, Craspedolepta veaziei, feeds on goldenrods, especially early goldenrod, Solidago juncea.  In June, adults move up the stem of the plant on to the branches of the inflorescence, feeding wherever they sit.  And they are mating.  And after they mate, the females has to lay eggs.  

But where?

Inside the flower heads of early goldenrod.  By the time the flowers bloom, the adults are nearly all gone (as shown in the graph; Craspedolepta veaziei used to be named Aphalara veaziei).  But the eggs are just getting started.  They hatch among the flowers inside the flower head where they are hard for enemies to find, and where they can feed as the flowers develop from bud through blooming into fruit.

Goldenrod species that develop later are not good hosts for these psyllids.  They need new flower buds when they are ready to lay their eggs.  Early goldenrod is the plant that has buds at the right time.

But by the time the flowers have become fruit, the psyllids are still immature.  Where do they go?  I think they spend the winter in the soil, perhaps associated with the underground parts of goldenrods, or perhaps just waiting, in a dormant state called diapause (it’s an entomological term – there are so many).  

The next growing season, large nymphs that have begun to resemble the adults, except for the absence of wings, feed on the stems of early goldenrod.  They seem to be well adapted for feeding and developing in conjunction with this host plant.  As far as I can tell, the plant is not significantly harmed by the psyllids.  Perhaps the late nymphs and adults, being exposed on the surface of the stems, are discovered by enemies often enough to keep these bugs from becoming too destructive.  

Further reading:

I apologize for not having a picture of this small insect, but you can get a sense of their appearance from photos posted on the web: https://inaturalist.ca/taxa/207542-Craspedolepta

Owen D. V. Sholes. 1984. Responses of arthropods to the development of goldenrod inflorescences (Solidago: Asteraceae). American Midland Naturalist 112: 1-14. http://www.jstor.org/stable/2425450

Some of the leaf is brown

(And the sky is not always gray)

Are there brown or gray patches on the leaves of your plants?  Does the leaf appear intact, but no longer green?  And is there no external sign of any insects?

Your plants might have leaf miners.  These are insects that have small and/or flattened larvae that live between the upper and lower epidermis of the leaf, mining (eating) the tissue in between.  It is an impressive adaptation, and there are several kinds of insects that do it (some flies, moths, beetles, and sawflies).

If the miners are still living in the leaf, the larvae might be obvious between the translucent layers of cells, or you can pry the leaf open and find them (what you do with them is up to you).  But it is quite likely that the miners have left the mine.  They might have pupated in the mine and emerged as adults.  Or they might have emerged as larvae and sought a place to pupate elsewhere.  

There are several species of leaf miners that feed on goldenrods, but one of the most common is the beetle Microrhopala vittata, the goldenrod leaf miner (in the family Chrysomelidae, the leaf beetles).  I found some of them in my examination of goldenrods back in the 1970s.  Naomi Cappuccino, studying goldenrods at multiple sites, including some near where I worked, often (but not always) found many of these beetles.  They were spotty, and their numbers fluctuated widely from year to year, for reasons that were hard to track down.  But sometimes, they were really abundant!

Walter Carson experimentally reduced beetle abundance in places where there were a lot of beetles.  When he did so, the goldenrods grew much better.  Where he left the beetles in place, the goldenrods grew poorly.  So these beetles matter, at least sometimes and in some places, to goldenrods.  

These mines on Solidago juncea (early goldenrod) certainly resemble those of the goldenrod leaf miner, though I’m not sure how distinctive the mines of the different species are.  I found them only on this tiny patch of stems, even though there are quite a few others within about 100 meters (though down a shady road – does shade affect the movement of the adults?).

I haven’t noticed any adults of the leaf miners, but it’s only June.  Stay tuned.

Further reading:

Naomi Cappuccino 1991

Mortality of Microrhopala vittata (Coleoptera: Chrysomelidae) in Outbreak and Nonoutbreak Sites 

Environmental Entomology 20 (3): 865–871

Walter P. Carson, Richard B. Root 2000

Herbivory and Plant Species Coexistence: Community Regulation by an Outbreaking Phytophagous Insect

Ecological Monographs 70 (1): 73-99