How Plants Work (2015)
9 Finding Love in a Sedentary World
SEX IS COMPLICATED, and it’s even more complicated for plants. Unlike people, they can’t go looking for Mr. or Ms. Wonderful with whom to start a family. Nor are their offspring able to pick up and move out on their own. Like other sedentary species, plants rely on the environment or living creatures to help out with matchmaking and house hunting. But plants have met this challenge in astonishing ways, often using unwitting animals, including gardeners, as a means of their own reproduction, dispersal, and survival. And they’ve been doing it for millions of years. While we can’t travel back in time to see how this started, we can look at some descendants of prehistoric plants.
In the Beginning
Woodland garden staples like mosses and ferns are ancient plants and structurally pretty simple compared to more recent botanical arrivals. Even so, they’ve obviously survived quite nicely despite their primitive nature.
As ancient as ferns are, they remain a popular plant choice for many gardeners, including me. Most of our ferns have volunteered to be part of our landscape, popping up along fence lines and among established shrubs. They arrived as microscopic spores, blown in on the wind or carried by raindrops and lucky to have landed where they found enough water to begin life. If you turn over the fronds of most ferns, you can find little brown bumps on the underside. These intricate patterns are spore nurseries, called sori. Mosses have tiny capsules containing spores; the hated horsetail has an asparagus-like structure containing spores at the end. (It’s this toxic stalk that’s sometimes mistaken for real asparagus by unfortunate foragers.) And liverworts (Marchantia species), one of my favorite little plants that’s equally loathed by nurseries, open tiny umbrellas from which spores are spread. Even cooler, the leaves of liverworts have miniature cups filled with clones (with the lovely name of gemmae) that sail off to new homes whenever dislodged by a raindrop.
Liverworts produce genetic clones (TOP) as well as genetically unique spores shed from tiny umbrellas (BOTTOM).
Liverworts provide a good example of the two ways that plants can reproduce: asexual clones and sexually produced spores (or seeds in most plants). While most plants can do both, some like orchids can only reproduce sexually. What’s the benefit of having more than one way to reproduce? After all, most animals reproduce sexually and only the more primitive members clone themselves.
Botanical Xerox Machines
Let’s consider cloning, or vegetative reproduction, first. It’s a fast and easy way of establishing territory, especially in environments where individuals might be isolated from one another. This is the strategy that many invasive species use: the mother plant provides hordes of storm troopers to conquer new environments. The clones are genetically identical to the mother plant and to each other, and given that an invading plant has been tough enough to survive and establish a new outpost, its clones will likely also be tough survivors. We use this characteristic in our own gardens, when we plant groundcovers. We don’t buy hundreds of plants, but just a few with the knowledge that they will spread and merge. By understanding the ways that plants spread vegetatively, we can use this information to propagate plants ourselves as well as predict how well new plants will spread in our garden before we plant them.
For millennia gardeners have shared their plants with other gardeners, and plants make it easy with their remarkable cloning ability. Many species simply make carbon copies of themselves; we call these multiple crown species or sometimes multi-trunked in the case of trees and shrubs. Whichever name you prefer, these species are easily divided by cutting the connecting roots and separating the crowns from one another, then replanting. Multiple crown species can create vast thickets of themselves. The largest known is a single quaking aspen clone spread over 100 acres in Utah.
In contrast, probably the most unusual method of cloning is leaf propagation. When most plants drop their leaves, these tissues die. But in some species the leaves develop roots on the detached end. If you have ever had a jade plant, you’ll have noticed that fallen leaves around the plant often root themselves. Other species, including African violets, snake plants, and begonias, send up new plantlets from leaf ends and cut veins. Kalanchoe, sometimes called maternity plants, form tiny plantlets along the margins of attached leaves; these fall to the ground and take root. It seems that most plants that successfully clone from leaves are succulent species or rainforest plants. In both cases, there is plenty of water available for the rooting process, either contained in the succulent leaf or in the environment itself.
We’re more used to seeing plants rooting from stems, either in the garden or as cuttings on our kitchen windowsills. Stems have a Tinkertoy type of structure consisting of nodes and internodes. The internodes (literally, places between nodes) are really nothing more than straws connecting nodes to one another. The nodes are wondrous little places containing meristematic tissue from which all kinds of new things can arise: new leaves, new roots, even new plants. This is why it’s important for gardeners to include nodes on their cuttings; without the node, all you have is a straw. (And because there’s no “this end up” sign on a stem, be sure to make the bottom cut at a 45-degree angle so you don’t root it upside down.)
The majority of my landscape shrubs have more or less upright, woody stems. But as the weight on these stems increases, they’re often bent toward the ground, where they can take root. Azaleas, spirea, honeysuckle, and many other ornamentals have this ability, and given enough time these rooted branches can survive on their own if they become separated from the original plant. Yet another desirable botanical trait for the frugal gardener to take advantage of!
But other plants in my landscape, especially groundcovers, have a different stem arrangement. The bright red runners of my native strawberry that crisscross the soil are horizontal stems, or stolons, that begin within an established plant. Along the runner, like beads on a string, are little plantlets that develop roots and start to grow if they happen to land on soil. The entire system is connected, so that roots from established plants provide water and nutrients to the runners as well as the developing, but unrooted, plantlets. Spider plants also have stolons. When planted in hanging baskets, the effect of all the little plantlets dangling from the stolons really does look like spiders.
When leaf-borne plantlets fall to the ground, they are ready to take root.
Mixed groundcovers hide a maze of rhizomes and runners underneath.
In contrast to my sun-loving, running strawberries, my landscape also has Oregon oxalis, a lovely understory native that pops up here and there as a result of a different stolon system. Oregon oxalis has underground stems called rhizomes, which also crisscross the landscape, though in a more secretive way. Only by moving aside the soil can one see the rhizomes and discover that their structure is much like that of runners. Many plants, including some truly annoying weedy species, spread by rhizomes.
While runners and rhizomes are recognizable as stems, other plant parts used for vegetative reproduction look more like roots. These stems in disguise include bulbs, corms, and tubers.
Bulbs are immediately recognizable to even the non-gardener, as everyone has peeled an onion at some point. Those layers of scales are actually modified leaves. So, the onion is just a flattened, white stem with fleshy white leaves that surround, protect, and nourish the bud at the center.
I’ve planted (and forgotten about) so many bulbs throughout the seasons in my garden that I’m assured of surprises all year round. Where only one flower might appear the first year, in subsequent years there are more and more blooms. Bulbs vegetatively reproduce by forming offsets, tiny bulblets nestled in between the scale leaves and the central axis. The offsets of some species, like bluebells, can flower the next year; others, like tulips, may need to develop for a few years before flowering. The bigger the bulb, the more food is stored in its leaves, and the bigger the flowers will be.
For this reason, many gardeners dig up their dormant bulbs every few years, removing the offsets and replanting them elsewhere so they don’t compete with each other. Digging animals do the same when they disturb the soil. Whether it’s a gardener or a squirrel, both are used by bulb-forming plants as the means to spread their clones throughout the landscape.
I’m a lazy gardener, so I tend to choose plants that take care of themselves. Thus, I use a lot of bulbs and corms in my garden. When I buy bags of tulips, daffodils, or crocus, I dutifully plant them to the proper depth, and then ignore them. They grow happily, bloom profusely, and spread prolifically. It’s easy to understand how naturally dividing bulbs can stay at the proper depth in the soil. But what about the seeds that drop to the ground, germinate, and plant themselves at the proper depth?
You might assume that animals bury them, either by accident or to store as food, but that’s a pretty unreliable way to ensure survival. Instead, the developing seedling eventually forms a tiny bulb (or corm) with specialized roots that actually pull the bulb into the soil. If you look at these roots closely, you’ll see that they look wrinkled. In fact, they look very much like a miniature version of the popular shrinking garden hoses. These contractile roots expand with water and then contract, pulling the bulb deeper and preventing it from becoming dislodged, damaged, or eaten.
It’s a great protective mechanism for ornamental bulbs and corms. Unfortunately, it works just as effectively with annoying garden weeds like wild garlic and yellow nutsedge.
Contractile roots protect bulbs by pulling them deeper into the soil.
Bulbs (TOP LEFT), tubers (TOP RIGHT), and corms (BOTTOM) are botanically different, but all of them can create new plants.
CORMS AND TUBERS
Like bulbs, corms have a flattened stem where roots emerge from the lower surface, but on top they have a rounded lump of storage tissue rather than neat, concentric layers of leaves. Garden favorites like crocus and gladiolus grow from corms, and their offsets (or cormels) can be found circling the bottom edge of the mother corm.
Tubers have no obvious base and instead have a swollen, lumpish appearance. They look like potatoes. And, of course, a potato is a tuber. Most of the tuber is storage tissue, except for the eyes, which are dormant buds. When the buds begin to develop, new plantlets are formed using the tuber’s stored reserves for food and water. So when you find that neglected potato that fell behind the shelf, you’ll understand that it’s so shriveled and flaccid because it’s supporting all that new, lush shoot growth. Just cut up that old potato and plant it!
Let’s Talk about Sex
Given all the benefits of vegetative reproduction and the myriad ways that plants can clone themselves, why do plants go through the fuss and bother of sexual reproduction? Well, anyone who enjoys breeding roses, tomatoes, or just about any other plant can tell you the answer to this one: it’s the way to create new cultivars. From the plant’s perspective, it’s strictly a matter of survival. By mixing things up, genetically speaking, plants can ensure that at least some of their offspring will survive in an ever-changing world.
Let’s look at one of my favorite plants, gardenias. I have tried for years and years to grow gardenias successfully as a houseplant. No matter what I did—no matter which window I chose or which fertilizer I used—I could never get the masses of pure white blossoms with their exotic fragrance. Sure, the plant would live, but it just wouldn’t flower. And, frankly, gardenias without flowers are pretty boring houseplants.
But several years ago a new gardenia called ‘Klehm’s Hardy’ became available. This cultivar can withstand moderate freezing, so now I’m able to grow my favorite fragrance flower outside. Through careful plant crosses, gardenia breeders were able to create a cultivar that can withstand lower temperatures than gardenias would experience in their native tropical and subtropical environments.
Without sexual reproduction, plants wouldn’t be able to develop resistance to insects, disease, and a whole range of environmental stresses. Plants tend to be rather lax in their selectivity of sexual partners and can be found happily hybridizing with other species, and sometimes even other genera. Many of these hybrids are aesthetically pleasing, resistant to stress, or have other characteristics that we enjoy. The London plane tree (Platanus ×acerifolia) is a hybrid between the Oriental plane tree (Platanus orientalis) and the American sycamore (Platanus occidentalis). This hybrid’s ability to withstand tough urban conditions and provide ample shade makes it a popular street tree. Crosses between raspberry and blackberry, plums and apricots, and mandarin oranges and grapefruits have given rise to delicious loganberries, pluots, and tangelos, respectively. And most of our favorite flowers are hybrid crosses, courtesy of nature and nursery alike.
The × in this name indicates the plant is a hybrid.
Why They’re Called Shrinking Violets
A person described as a shrinking violet is pictured as shy and retiring. This is hardly an apt description of real violets, which are such prodigious seed producers that many gardeners consider them weeds. It’s likely the name arose after observant gardeners noticed that violets not only display their small, showy five-petaled flowers, but bear hidden flowers as well. Found close to and sometime beneath the soil surface, these flowers never open: they are cleistogamous, which translates colorfully to closed marriage. As the name implies, the purpose of cleistogamous flowers is to produce self-pollinated, fertile seeds, which quickly fill in gaps around the parent plants and create dense patches of violets.
The phenomenon isn’t limited to violets, either. Several other well-known garden plants have this interesting adaptation, including many grass species and legumes such as peas, beans, and peanuts. Cleistogamy is a common survival strategy for plants in harsh environments. They’re always able to reproduce even when they’re all by their lonesome. And now you have a fun new word (klī-stŏg’-ă-mē) to trot out at your next social function.
Some violet flowers can only be seen by digging up the plant.
We know that animals sometimes hybridize: ligers from lions and tigers and mules from horses and donkeys, for example. But these hybrids are usually unable to reproduce, and therefore they’re just an evolutionary dead end. Hybrid plants, on the other hand, are often fertile and can reproduce themselves. This makes the family trees of many garden favorites, like rhododendrons, infernally complicated. The parentage of one of the rhododendrons I used for my doctoral research consisted of one species, several cultivars, and a helpful dose of unknown.
Not all hybrids are desirable, at least as far as gardeners are concerned. Allowing certain squash varieties to cross-pollinate, for example, can result in some truly vile produce. Many gardeners who grow heirloom vegetables and collect the seeds go to great lengths to avoid exposing their garden gene pool to undesirable outside influences, like a shotgun-toting father might protect his teenage daughters. Uncontrolled plant hybridization can result in muddy flower colors, lack of fragrance, and other characteristics that gardeners find unappealing. But the purpose of plant promiscuity is to constantly produce new combinations of genes that might help the next generation survive future challenges—not to please people. When hybrids do both, however, they ensure their future survival by using gardeners as the means to that end.
THE BIRDS AND THE BEES
We’ve discussed the benefits of sexual reproduction that cloning can’t confer to plants, but we still haven’t mentioned how it works. It’s time for the talk.
Birds and bees, bats and butterflies, wind and water: plants use all of these as transport systems for pollination. Conifers, the oldest seed-producing plants, set their pollen on the wind in search of receptive female cones. In fact, any plant that produces clouds of yellow pollen, like pine trees, grasses, and ragweed, belongs to a wind-pollinated species. In grasslands, dry pine forests, and other environments where species diversity is low but plant numbers are high, wind is a low-cost method of sperm delivery.
In more complicated ecosystems, where many species jostle for space and individuals of the same species may be few and far between, other methods of pollination are needed. This is where plants use their flowers to entice animals to become willing—if unwitting—delivery services. As we’ll see, it’s a more expensive process for the plant, but it increases the likelihood that pollen is delivered to another flower of the same species.
For animals to willingly act as plant matchmakers, they need to receive some kind of payment. Plants reward their pollinators by providing food (nectar and pollen), nest building materials (oils and waxes), heat, or other necessities. To attract pollinators, flowers use color and odor as advertisements, and these odors and colors are often fine-tuned for specific pollinators. When we gardeners understand how birds, bees, butterflies, and other pollinators see the world, this can help us select flowers that will attract these garden visitors.
I have a lovely ruby-red glass hummingbird feeder hanging in my south-facing garden, and hummingbird lovers know that red is a color easily seen by birds. Fuchsias, columbine, hibiscus, and other red and orange flowers are bird magnets, often providing so much nectar for their hungry visitors that this sugary treat drips from the flowers. The flowers themselves tend to have tubular shapes to accommodate bird beaks and tongues, and they are sturdy enough to withstand buffeting by these relatively heavy pollinators. Birds don’t have much of a sense of smell, however, so bird-pollinated flowers generally lack fragrance.
Moths and butterflies also seek out red and orange flowers, but in contrast to birds they are strongly attracted by scents. The night-blooming flowers, like jasmine, don’t waste energy on colors but are a luminous white with heady perfumes to attract nocturnal moths.
Bees and some other insects don’t really notice red at all. Their best vision is down in the blue end of the spectrum, so blue and purple flowers like foxglove, lupine, and delphinium are bee attractors. Interestingly, bees also see into the ultraviolet region, which isn’t visible to us. But many bee-pollinated flowers have ultraviolet guides, which act like miniature landing strips on sturdy petals, all leading to the center of the flower. White flowers, in particular, often look very different under ultraviolet light and obviously are catering to bees as pollinators.
Bees can see ultraviolet light, which reveals floral landing strips. The photo on top shows what we see. The photo below was taken using an experimental filter that allows us to see what bees see.
Of course, there are other garden pollinators, including bats, beetles, flies—and you! What’s your favorite garden flower? The first brilliant yellow daffodil of spring? Fragrant David Austin roses? Masses of electric blue mophead hydrangeas? Exotic night-flowering jasmine? Simple, sophisticated calla lilies? Whether it’s the fragrance, color, or shape that appeals to you, plants use that to their advantage. Even though many of our ornamental cultivars are sterile, any time we press our noses into one flower after another we’re transferring pollen. Along with our penchant for dividing and sharing garden and house plants, humans are possibly some of the best reproductive aids in the botanical world.
Outfoxed by Foxglove
When I’m giving talks to gardeners I sometimes show a picture of a round purple flower that’s heavily speckled and ask my audience what’s wrong with the plant. Most people think it’s a disease problem, but it’s actually just another freaky flower phenomenon that’s rooted in the past. Primitive flowers were radially symmetrical (round), much like current day sunflowers, magnolias, and roses. As insects and other airborne critters discovered the all-you-can-eat buffet of protein-rich pollen and sugary nectar, some flowers took advantage of this free shipping option for pollen by changing their shapes. Bilaterally symmetrical flowers (those that have right and left sides) are able to force pollinators into places where they are most likely to pick up pollen accidentally and transfer it elsewhere.
Foxglove has bilaterally symmetrical flowers and bees crawl into the tubular blossoms to forage for nectar. Every once in a while, however, a flat, disc-shaped flower will show up on a foxglove spike, usually at the terminal end. This phenomenon is called peloria and is simply a reversion to the primitive radial form. You can frequently find peloric flowers in orchids, and even Darwin found them in snapdragons.
This foxglove flower has reverted to a primitive form.
SPREADING THE WEALTH
It’s obvious that gardeners not only have green thumbs, but they’re under green thumbs in terms of obeying their plants’ reproductive bidding. But plants have one more difficulty to overcome: how to disperse all of those seeds that have formed after successful pollination. Like pollen, seed dispersal relies on natural forces as well as animals.
Some seeds are spread by water. The most massive seed of all, the coconut, is moved by ocean currents far from the shoreline where the mother palm stood. Plants growing in ponds and along stream banks can also take advantage of water currents to float their offspring away. Others set their seeds on the wind using various buoyant structures to keep the seeds aloft as long as possible. The parasol seeds of dandelions, the rotary blades of maple achenes, and the cottony billows of poplars allow the offspring to establish their own territory and lessen competition with their parents and siblings.
But wind and water are unpredictable methods of transportation. Seeds may end up in areas where they can’t survive. Animals, in contrast, not only move seeds but often provide a nice dollop of fertilizer to go along with them. Therefore, many plants have developed tasty and nutritious wrappings for their seeds, as inducements to animals to serve once again as unwitting chauffeurs.
The Hottest Thing for Your Garden
Many gardeners recognize anthuriums, though only those in tropical climates can grow them outside. Other members of the arum family (Araceae) include calla lilies, jack-in-the-pulpit, skunk cabbage, philodendron, and the awesome titan arum. One thing these plants have in common is a spadix, which consists of a multitude of tiny flowers clustered on a sturdy stem. This suggestively shaped structure has given rise to some snicker-worthy scientific names, like Amorphophallus titanum, which means giant misshapen penis. Ahem.
Indeed, the spadix plays an unusual role in reproduction for many plants in the arum family. It produces heat—a lot of heat—through an unusual biochemical pathway. Some flowers have been measured to reach over 100°F. The heat helps volatilize odors that attract pollinators, which unfortunately tend to be beetles and flies that like rotting meat. Skunk cabbage is an aptly named arum and one that explains why some arums aren’t a popular gardening choice.
Skunk cabbage flowers generate enough heat to melt snow.
Titan arums are visually stunning and equally malodorous.
This heat production has other roles for arums in cold climates: it provides a haven for pollinators (like the heliotropic plants in the arctic) and allows arums to get a jump start on early spring growth by melting the surrounding snow and ice.
Fruit eaters know to avoid eating green fruit, which is not yet ready for seed dispersal.
Plants use color and fragrance to advertise the edible rewards available for animals willing to take the kids for a ride. The hard green tissues of unripe fruits gradually plump, sweeten, and turn all colors of the rainbow, signaling to hungry animals that the kitchen’s open and dinner is served. This ripening time is necessary to ensure that the seed, tucked safely away in its tough seed coat, has reached maturity and can leave the mother plant.
Species that produce relatively small seeds, from berries to apples to melons, are adapted to having their offspring ingested right along with the sweet pulpy fruit. The seed coat resists complete digestion, but it is etched by stomach acids before the seed is finally expelled, ready to germinate and use up those conveniently deposited nutrients. Other seeds might have their coats scratched (or scarified) by wind, water, sand, or cold temperatures. Still others, like some pine species, need trial by fire to pop open the cones where the seeds are trapped.
Plants protect their seeds from animals with good reason: they are highly nutritious, containing protein, carbohydrates, and often fats. Think nuts, legumes, and grains. So some plants, especially those with larger seeds, have turned to more sinister means of protection. Apricot pits contain chemicals that turn to cyanide, and castor beans contain ricin. Many of the unique poisons made by plants are used to defend their offspring from seed-eaters.
From the most primitive moss to the tallest conifer and to the most elegant orchid, plants have managed to use every body part and exploit animals as well as the forces of nature to ensure their spread into every environment, on every continent except Antarctica. So when you’re in your garden this year, pulling those annoying spring weeds, enjoying those first summer blueberries, or dividing your irises in autumn, think about this amazing accomplishment by a life form that’s literally rooted in place.