Moss to Maple

a lightning tour through the evolution of land plants

A maple tree is clearly very different from a moss, yet over a long period of time, and step by step, early moss-like plants evolved to trees, including maples. The fossil record tells us about some of these steps, and experts have agreed upon the probable steps where the fossils don’t tell us. The story centers on solving problems involved with the invasion of land from the sea or fresh water.

Hundreds of millions of years ago, green algae began to colonize land. They already had the mechanisms of photosynthesis: using light to combine carbon dioxide and water into nutritious carbohydrates. And they already had sexual reproduction, which produces new combinations of genes every generation and thus the variation necessary for evolutionary change. Experts say, and there are hints in the fossil record, that the very early land plants formed associations with fungi that provided nutrients, a mutualistic association that almost all land plants have maintained. So those early terrestrial plants were off to a good start.

For sexual reproduction, however, those early plants were entirely dependent on water: eggs and sperm were released into water, where sperm could swim a few centimeters to find a drifting egg to fertilize. That’s OK for plants living in water or even in damp soil, but it won’t work on dry land.

The first land plants are thought to have been more or less moss-like: small plants growing close to the ground. If conditions were not wet enough, these early plants had to wait for sex until conditions improved. (Although most modern mosses still have to wait, as did the early plants, a few are reported to use springtails, mites, or flies to transfer sperm to eggs, but few such animals were present in the early days of the invasion of land).

But even if sperm could swim to an egg, the resulting zygote would be still on its own in a potentially dangerous environment. Somewhere along the line, although sperm were released and still had to swim, eggs began to be retained in special structures on the leaves of the parent plant. Fertilization then occurred within these special structures and the zygote received both protection and some nutrition during development. In mosses, that zygote stays on its mother and grows into a new individual (called a sporophyte) that looks different from its mother and eventually produces spores. Spores are single cells inside a tough coat that disperse on the wind and, if they land in a good spot, grow into new mosses (called gametophytes, because they produce gametes). Thus, the generations alternate between gamete production and spore production.

One generation grows atop the other: a sporophyte has grown from an egg retained and fertilized at the tip of the green moss plant (a gametophyte). Spores will disperse from the capsule and start new green moss plants. Photo by Bob Armstrong

There was still the problem of needing water for the sperm to swim to an egg. The fossil record is poor at this point, but clearly, at some point, moss-like plants began to produce two kinds of spores on their sporophytes: small, male spores with sperm and big, female spores with eggs. The big spores did not disperse but were retained on their mother sporophyte’s leaves, where they received protection and nutrition. The small spores ultimately developed more protective covers; they traveled on the wind and found special landing places near the female spores, where they could fertilize the eggs. In some cases, that special landing place is a droplet of fluid, close to the egg, that engulfs the arriving male spore (if it belongs to the same species as the egg) and pulls it in. That is how pollination came about. The big, female spores began to stock nutritional material inside, for the growth of the embryo, and that was the beginning of the evolution of seeds.

The early land plants (and modern mosses) were often at risk of desiccation. Protection from drying out came with an impervious cuticle over the outer surfaces. But complete imperviousness would not only prevent water loss but also prevent carbon dioxide (for photosynthesis) from entering. Conveniently, surface pores called stomata, which can open and close, allow the entrance of carbon dioxide and help control water loss.

Mosses don’t have a very good system for transporting water from one part of the plant to another. But some of those early land plants developed a vascular system for water transport. This is built from lined-up, hollowed-out cells with reinforced cell walls (called xylem; concentric bundles of xylem became known to us as wood). The resulting channels connect to the stomata. When a stomate is open, water vapor is lost faster than carbon dioxide comes in. This is called transpiration (in parallel with evaporation from a surface). Because water molecules in the vascular channels cling together, transpiration from stomata pulls columns of water up through the channels. No energy expenditure needed, except to open or close stomata. These channels deliver water to leaves and other parts of a plant. The walls of the channels are reinforced to prevent collapse.

The early plants did not have true roots, just small anchoring fibers. True roots did not develop until there was a vascular system to deliver water to them. Conversely, without roots, the nascent vascular system could not draw much water from the soil. But, somehow, early vascular plants did develop roots of some sort, which not only anchored the plant to the soil but also transferred soil water and nutrients to the plant above and to the roots themselves.

Development of a vascular system happened in sporophytes, not in gametophytes, which initially needed to stay small for sexual reproduction (but later began to stay on the sporophyte, as noted above). Those stiffened channels also provide mechanical support, so vascular plants could grow tall. Being tall is often an advantage when competing for light.

The radical remodeling of cells needed for the development of a vascular system in sporophytes may have been facilitated by the fact that sporophytes have two sets of chromosomes (compared to gametophytes and gametes with just one). That means deleterious mutations in one set of chromosomes could be masked by normal genes on the other set.

Thus we arrive at a hypothetical ancestral plant, with some kind of pollination system that does not depend on water, a primitive seed with nutrition for an embryo, a vascular system, roots, and the ability to grow tall, with mutualistic fungal associations. The stage is set for the evolution of seed plants. There are basically two kinds of seed plants: all the flowering plants, including maples, oaks, shooting stars, and lupines, whose seeds are enclosed in layers of maternal tissue, and the conifers and their relatives, whose seeds are borne on the surface of leaves or scales. The evolution of these two branches of the plant evolutionary tree makes another complex story…


Stories under the bark

a bristly millipede and a selfless spider

A well-known local photographer and naturalist lifted a flap of hemlock bark and found some interesting small creatures. There were several tiny millipedes, only two or three millimeters long. Unlike most millipedes, which are plated with hard covers on each segment, this kind was covered with bristles (of undetermined function). According to experts, this is a species of Polyxenus, probably Polyxenus lagurus, a widespread species. At the rear end of the body are two tufts of detachable, hooked spines—a sort of grappling hook–that form a defense against predatory ants and spiders. When attacked, the millipede swings its rear end toward the predator; the spines detach upon contact and cling to the attacker; when an ant tries to groom off the hooks, it just makes matters worse, as it gets entangled and incapacitated in a snarl of spines. Of course, there is somebody somewhere who beats the system: a large tropical ant subdues a polyxenus by stinging it before it can use its hooks.

Photo by Bob Armstrong

Many millipedes live in moist places (some get to be a foot long!), feeding mostly on plant debris. However, the little bristly ones feed on lichen and algae on bark and rock surfaces, where they are often exposed to the risk of desiccation, and their foods also get very dry, providing little dietary water. But they have a thrifty way of conserving water: by absorbing water vapor though the walls of the rectum (the last part of the intestine). Moisture in the feces produces water vapor and high humidity in the rectum; because there is more water vapor there than on the other side of the rectal walls, it passes (by osmosis) through the walls to parts of the kidney that lie tight against the rectal wall. A shriveled, well-dried polyxenus can rehydrate in just a few hours, if it happens to find a bit of moisture. Rectal water conservation has evolved independently in a few species of several unrelated kinds of small invertebrates, including some beetles, fleas, cockroaches, mites, and isopods.

The other interesting creature under that flap of bark was a handsome spider, identified as a species of Callobius, a genus that occurs mostly in North America. They live under bark and stones or in the leaf litter. The common name is hacklemesh weaver or lace-weaver spider, named presumably for the loosely-woven, fuzzy-textured mesh with which they make a somewhat disorganized funnel web. The males overwinter as immatures, molting to adult form in spring; after mating, they die. Females, however, are found all year round, and may live two or more years. They lay eggs, often in their webs, with several dozen eggs in each cocoon. What happens when those eggs hatch is not yet known, but close cousins of these spiders (in the genus Amaurobius) do something unusual, so maybe Callobius does too.

Photo by Bob Armstrong

An Amaurobius female lays one clutch of eggs and guards them until they hatch. She is induced by her first batch of offspring to lay another clutch of eggs, which are usually infertile. These are called trophic eggs, and they are eaten by the first offspring. Then the female vibrates her web, which stimulates those offspring to eat their mother! This is called matriphagy, an extreme form of parental care. Motherhood may have a high cost, but there is a payoff: Offspring that are fed on trophic eggs and on the mother get bigger and probably survive better than those that are not. They stay together for a while, hunting cooperatively to subdue prey. Females that provide trophic eggs and subject themselves to consumption by their offspring have higher reproductive success than those that don’t.

The habit of producing trophic eggs has evolved independently in certain species of unrelated animals, including tree frogs, ants, crickets, stingless bees, and snails. In some species, trophic eggs are the only food the newly hatched offspring get. Young mackerel sharks develop within the uterus of the mother, who produces numerous eggs. Early-hatching offspring commonly eat undeveloped eggs while still inside the mother; in some cases, this habit progresses to cannibalism on late-hatching embryos.

Similarly, matriphagy, usually with lethal consequences for the mother, has evolved independently in certain insects, nematodes (round worms), scorpions, as well as spiders. There is said to be one vertebrate in which the young eat part of the mother, without lethal consequences; in a species of small amphibian known as caecilians the young eat only the skin, which the mother can regenerate.

Who would have guessed that such strange and fascinating stories would be lurking under a flap of tree bark!

The peace of wild things

when despair for the world grows…

There’s a poem by Wendell Berry, written over five decades ago, called The Peace of Wild Things. The poem has been reprinted many times and also set to music. At a recent concert when the song was performed, I was asked to make an introduction, using my experience as a local naturalist—a personal statement related to the thoughts expressed in the poem—despair and grief at the state of the world and finding solace in Nature. With great trepidation, I agreed to do this, although this task was much harder than writing a scientific paper or giving a class lecture. So here goes…

Like the poet in the song, I am very pessimistic about the future of the natural world. The things we proudly call civilization and progress have unleashed a monstrous wave of destruction over the planet. The human population has been and still is increasing at a phenomenal rate, creating social tensions as well as prodigious ecological damage. With little or no regard for the often predictable consequences, human activity is wreaking havoc everywhere—cutting forests, plowing prairie, paving ponds and marshes, overharvesting and overgrazing, extracting material resources and dumping what’s unwanted, polluting land and ocean and air, warming the climate…causing one of the biggest extinctions in the history of the world.

There is clearly reason for grief and despair. No use waiting for government or bureaucracy to act, and I think collective human behavior is unlikely to change very much. It all comes down to individuals.

But distressed minds do not think clearly; despair must be temporarily set aside behind a mental curtain, in order to live productively and constructively, perhaps even doing some small things to slow the pace of destruction. In a search for equilibrium, I seek comfort and peace where it can still be found.

Even as a child, I found comfort in walking in woods or meadows, doing nothing in particular, just looking and listening. Mind you, I didn’t know anything; I reckon I could tell a maple from an oak, and a robin from a blue jay, but that’s about all. Now I know a bit more, and I still walk outdoors, in forest or meadow, virtually every day, looking and listening.

The best kind of walk for me, either solo or with a like-minded friend, is one of noticing things in my surroundings, finding little stories of interest. The surroundings are not just scenery. I am intently focused, so it’s a kind of meditation, but that focus is directed outward. I love the little stories I find—here’s where a weasel popped up out of the snow, bounded a few yards, and dove back under, presumably hunting voles. Here are the tracks of a junco near the web of a winter spider, where it may have picked bugs from the web. And here is a bumblebee, sleeping in a flower. Look how happy the lichens are, after that rain! There’s a nest of a black-backed woodpecker, and the noisy young ones are just now fledging! The ruby-crowned kinglets are here again, and singing my favorite song.

I take other kinds of walks, of course—some might be mainly for exercise, or for sociality with friends, or for time to brood about what to make for dinner…But even those walks bring some inner peace, because I know the other lives are there. I am hearing a chorus of subtle voices, not with my physical ear, but internally. It is a chorus of tardigrades in the mosses, of voles under the snow, of sticklebacks in the ponds, of shooting stars waiting for a bee… On such walks, I ‘hear’ them without paying focused attention; I just know that they are there. In a different way, that can be as comforting and important as listening to a Schubert sonata or a Haydn symphony or a Bach motet.

By focusing on little stories, listening to the subtle voices, and just knowing they are there, I find a feeling of connectedness to the other lives in natural communities around me, making me part of it all. As another song says, Because you are listening, you’re part of the song! That fosters some inner peace, restedness, and renewal.

I’ll end here with a small story. After my husband (and canoe partner) died, I was determined to keep on canoeing. I bought a solo canoe, and for many years I made solo trips to the Boundary Waters and Quetico, staying out for a week or more at a time. Each trip was an experience of total immersion in the here and now, a kind of strenuous meditation; for that time, there were no past or future concerns. No motors, very few people, an occasional bear. One day I pulled up to a portage, unloaded my boat, and was about to swing my canoe up on my shoulders, when I heard a bird song from the top of a nearby tree. Not a robin….so I looked up. At the top of a white birch tree with brand-new shiny green leaves, backed by a cerulean sky, there was a male scarlet tanager! Oh my! That vision and that song carried me over the portage and a long way on, and they’re with me still.

Peace be with you!

A plant called pipsissewa

the intriguing story of a small flower

The rapidly lengthening days of March are an annual pleasure for us all, I think. They also make some of us fidgety—eager for spring and summer to arrive. As I write this, however, the early morning temperatures at my house are around five degree Fahrenheit, which suggests that I might be a be over-eager with my fidgeting.

Nevertheless, my thoughts turned to a small shrubby plant in the forest understory, one that blooms in summer. So I decided to write about it, even though it is not exactly seasonally appropriate.

This little plant is called pipsissewa (or sometimes prince’s pine, though it has nothing to do with either princes or pines); its scientific name is Chimaphila umbellata. A friend called to my attention a small patch of it one summer day, as we prowled along a forest trail on the lower slopes of the west side of Mendenhall Lake. I think I have since seen nonflowering specimens in several other spots. However, the field guides and books on the flora of Alaska present somewhat conflicting information on how to tell this species from leatherleaf saxifrage when no flowers are present, so I can’t be sure.

Photo by Bob Armstrong

Then it dawned on me that I knew absolutely nothing about this plant except that it belongs to the wintergreen family and, like its relatives, pipsissewa has shiny evergreen leaves. So I dug up some information. This species grows in the mountain west and across the boreal forests of North America and Eurasia, and research information comes from eastern North America and northern Europe.

When summer finally rolls around again, pipsissewa produces a few pinkish-white flowers, more or less saucer-shaped. The flowers are said to have a faint aroma, and they produce nectar. Pipsissewa flowers later in the summer, after many other species have finished flowering. It is pollinated primarily by nectar-collecting bumblebees. Many of the pollinating bees are males, which are produced late in the season. Although they are less efficient pollinators than the (female) worker bees, they gain a source of energy for flying around looking for females with which to mate (those queens will produce next year’s broods of workers). The flowers are self-compatible, meaning that pollen originating on a given individual can fertilize ovules and produce seeds on the same plant. But it usually takes a bee to accomplish the pollen transfer from the anthers, where pollen is produced, to the female-receptive parts of the flower. Pollen grains are produced in clumps of many grains stuck together, so many of the seeds that develop from each pollination event are likely to be full-siblings with the same father.

Pipsissewa produces large amounts of very tiny seeds that hold little nutrition for a developing seedling (as is true for other wintergreens and for orchids). The seeds are buoyant and disperse on vagrant puffs of air in the understory, but most seeds probably don’t go very far. One study found that germination was better if seeds landed near an established adult plant and in more acid soils.

The miniscule size of the dust-like seeds means that some other source of seedling nutrition is needed, since so little is stored in the seed itself. Pipsissewa and other plants with dust-like seeds are ‘fed’ via fungal connections to other plants that are well-established, already photosynthesizing carbohydrates, which the fungi transport to pipsissewa and other plants; these fungi also supply nitrogen and other nutrients to the growing seedling. As the young pipsissewa grows to adulthood, it apparently becomes less dependent on the fungal connections for carbohydrates but still obtains other nutrients that way.

The fungal connections are called mycorrhizae (fungus-root), which have featured in my essays many times. Most plants, including ferns and mosses, in our forests have these connections, moving nutrition from plant to plant. Some plants are entirely dependent on their mycorrhizae for growth and maintenance, others are only partly or temporarily dependent, while still others are much less dependent and may serve chiefly as suppliers. I suspect that we would not recognize our familiar forests if they lacked these fungal connections; altogether, one could say that fungi make our forest what they are!