The bright, abundant flowers of goldenrods are a conspicuous part of their life cycle. Pollinators visit the flowers for nectar and pollen, and as they move from flower to flower and plant to plant, they spread pollen. Pollen grains contain sperm cells that find their way into the ovule within the ovary, where there is an egg nucleus. The merging of sperm and egg nuclei produces the first cell of an embryo, which will remain dormant inside the seed until the time and place are right for germination.
We will revisit pollination and fertilization when they are actually happening in the summer.
In sexually reproducing organisms, species are usually defined as groups of organisms that can breed with each other. For most goldenrods, that’s exactly what they do: reproduce with members of their own species. But quite a few species manage to breed with other species and form hybrids. Apparently, pollen moves easily between them, and then the sperm and egg cells are able to fuse to produce viable offspring. Hybrids occur rarely enough that the species remain mostly separate, but they occur often enough for botanists to notice.
Among those botanists was Merritt Fernald in the first half of the 20th century. He compiled the eighth edition of Gray’s Manual of Botany in 1950, a century after Asa Gray had produced the first edition while he was on the faculty of Harvard University. Fernald recognized sixty-nine species of Solidago (excluding Euthamia, which was then a subgenus of Solidago, but is now a genus of its own). Of those sixty-nine species, Fernald stated that twenty-seven form hybrids with other goldenrods, and he listed the species with which each species did so.
We have learned a lot about goldenrods in the last seventy years, but the species in Fernald’s compendium are nearly all still recognized as “good” species in sources such as Flora North America, though some of the names have changed. I think his list of species is worth examining further.
Among the species in Gray’s Manual that were said to hybridize, I think it’s interesting that some species hybridize with only one other species, while others hybridize with as many as eight species. I expected that hybridization would occur with closely related species, and it often does. Using the subsection classification within Solidagopresented in Flora North America, eighteen species hybridize within their own subsection (presumably species that are closely related), but seventeen can cross with species beyond their nearest relatives.
Canada goldenrod (S. canadensis) and wrinkled-leaf goldenrod (S. rugosa) are the champions, hybridizing with eight and seven other species, respectively. Next is silverrod, Solidago bicolor, with six hybridization partners. Silverrod has white petals around yellow stamens and pistils, suggesting that the difference between white and yellow petals does not prohibit crosspollination. Who knew?
According to the notes in Gray’s Manual, hybridization is not always symmetrical. Was this a mere oversight, or did botanists know that the pollen of species A could fertilize species B, but the pollen of B could not fertilize species A? This kind of asymmetry is not as strange as it might seem. There are complex interactions between pollen grains and pistils. Is the stigma receptive, can the pollen grain germinate (yes, it germinates), can the pollen tube grow down the style and into the ovule (yes, there is a tube for the sperm cells to follow), can the sperm enter the ovule, then find and fertilize the egg? Any of these might work in one direction, A to B, but not the other, B to A.
Some of the hybrids alleged by Fernald seem, on the face of it, highly improbable. The stems of Solidago caesia, blue-stemmed goldenrod, grow at an acute angle to the ground, spreading laterally rather than vertically. Their flowers form in clusters in the upper axils of the leaves, dotted along the stem. Yet this species is supposed to hybridize with Solidago canadensis, a tall species with vertical stems and a large branching inflorescence at the top of a conspicuously tall plant. I dismissed Fernald’s claim – until I saw a plant on the Cornell University campus that was only somewhat bent from vertical, and that had both axillary and terminal flowers. That plant made me stop and pay attention to the malleability of goldenrods.
Researchers have learned quite a bit about goldenrods in the seven decades since Fernald published his edition of Gray’s Manual. A few of the hybrids turned out not to be hybrids at all, but merely reproduction within a highly variable species. And some species have experienced hybridization in the past that is not obvious by merely examining the structure of the plants. How do we know? Chromosome numbers. That’s a topic for another post.