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Do Plants Sleep? Botany By Moonlight

Articles: Do Plants Sleep? Botany By Moonlight

Fall 2023

This article and associated photography is excerpted and reproduced from Nocturnalia: Nature in the Western Night by Charles Hood and José Gabriel Martínez-Fonseca with permission from Heyday © 2023.


For many of us, we’ve been taught to fear the night, and so we try to keep it at bay with deadbolts and security lights. Yet nature does not stop when the sun goes down: plants bloom, spiders spin, treefrogs call, and owls hunt. One kind of nectar-eating bat likes to visit hummingbird feeders; another species, the pallid bat, patrols slowly, ready to grab beetles and scorpions right from the ground.

José Gabriel Martínez-Fonseca and I worked on this project not only to learn more about nature after dark ourselves, but also to photograph nocturnal nature in ways that showed its beauty, not how fierce or menacing it can be. We are very happy with our bat portraits, for example, and also shots like the one that shows the ethereal glow of a queen of the night cactus.

We are especially pleased to be able to include examples of urban nature in the book. Don’t tell anybody, but the shot of a pollinating sphynx moth was taken in my front yard, right in the middle of suburbia. (“If you plant it, they will come.”) Our best fox and coyote shots came from automated cameras set up at José’s house in Arizona. Too late to make the book, but I finally got a great night shot of a praying mantis patrolling my yard’s purple sage.

If we had a motto, it would be, “Nature at night—try it, you’ll like it.”

– Charles Hood

Chapter 3: Do Plants Sleep?

Leaves are verbs that conjugate the seasons.

—Gretel Ehrlich

Nature thrives using the barter system. You give me some calcium, and I’ll give you some bones and antlers. You give me some sunlight, and I’ll give you an ear of corn. In old primary school charts, this was shown as a pyramid—plants on the bottom, leading up to a puma or bald eagle at the top. (Whenever there is a shark or a pack of wolves in a nature example, it always seems as though they’re a proxy for a human, since we know in our deepest hearts that if there is such a thing as an apex animal, it has to be us.)

Although a better shape than a pyramid might be a honeycomb or tangle of yarn or even overlapping Venn diagrams, it is true that plants outnumber pumas, and just as we learned in school, it is also true that (almost) all plants, algae, and cyanobacteria perform photosynthesis. To do their alchemy, plants start by taking in carbon dioxide and water from the air and soil. Within the plant’s cells, the water is oxidized, meaning that it loses electrons, while the carbon dioxide is reduced, meaning that it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose. The plant then releases the oxygen back into the air and stores energy within the glucose molecules.

The creosote in this view is not static; branches swell and move at night. Photo: National Park Service

This process helps create and maintain the oxygen content of the atmosphere.

After dark, even though there’s no sunshine and hence no direct photosynthesis, plants still have work to do. Botanist Peter Thomas points out that “trees and other plants are not dormant or ‘asleep’ in the cool of the night. Trees do most of their growing at night, perhaps because water stress is lower.” Coastal redwoods are moving water around in significant ways, and have the stomata (gas valves) open in their leaves, perhaps to help pull water to the topmost layers and also because nighttime fog condenses on the needles and enters the system from the top down, rather than roots-up as it is in most water circulation systems.

So much is going on that a plant’s shape changes. Scanning with lasers shows that many plants bend and pulse at night. In the case of one magnolia tree, measurements revealed that there was a cycle of movement three times a night, with each cycle taking about four hours. As water circulates in plants, the branches stiffen and swell, and in a study of desert creosote, on a windless night even dead branches changed position, presumably due to small changes in the humidity of the air. In the morning, some plants reset their branches back to their original positions and others didn’t, ending up “caught in the act” by the laser surveys.

Plant Clocks: How Do They Know What Time It Is?

Light can be measured by both its intensity and its hue, meaning how strong and blue it is (which implies stark noon light) or how soft and amber (the “golden hour,” beloved by cinematographers).

All living creatures have genetic coding that creates a circadian rhythm, and that “on/off switch” is tied to our perceptions of the quality of light. We “know” what (and hence when) sunset is, subconsciously and continually. That is why computer screens at bedtime are so toxic to good rest, and why artists like Thomas Kinkade know that painting a tidy cottage with a golden, firelit glow in the windows creates a sense of comfort.

Hooker’s evening primrose is pollinated by crepuscular bees and nocturnal moths. Photo: Charles Hood

In plants, the light sensors are called phytochromes, and those structures are also found in bacteria and fungi, and even in single-celled prokaryotes. Phytochromes regulate the germination of seeds, the synthesis of chlorophyll, the elongation of seedlings, and the timing of when a mature plant produces flowers.

They work in concert with sugars inside the plant to track time and tell the plant when it is appropriate to start photosynthesizing.

In one recent experiment, researchers manipulating the sugars in a lab caused the plants’ clocks to become misaligned by several hours a day. (If you’ve ever had jet lag, you may know how they felt.)

As plants “wake up” in the morning and respond to sunrise, the collective change in their rate of transpiration is visible from space. Sensors mounted on the International Space Station use a radiometer to measure the thermal infrared energy emitted by plants. These data can be used to create color-coded maps, and as the morning progresses, the maps show vegetation responding to daylight from east to west, like a giant carpet being unrolled across New England and the Great Lakes. These plants have had a busy night and, through the magic of photosynthesis, will be fueling up for the days and nights ahead.

Pollination after Dark

It is not enough to grow and look pretty; plants also need to be pollinated for the health and genetic diversity of their seeds. Some use wind to achieve this, some use hummingbirds, some use bees and other insects, and some use all the tools of the night. Flowers that open at night are often white and showy—in dim light, this is not the time to be subtle—and usually offer strong olfactory clues as well.

Most night-blooming flowers are pollinated by moths. In the West, plants that have evolved to solicit nocturnal pollination include evening primrose, golden columbine, rock trumpet, ajo lily, the four o’clocks, and the evening-blooming morning glories. Datura is another night bloomer and common roadside plant. It has white, trumpet-shaped blossoms. You probably have seen it, since it can stay open during the day and grows easily and prolifically in dirt lots and along the margins of rural roads. You can see the flowers even if you’re going sixty miles an hour. Their intended audience is not us but moths, such as the white-lined sphinx moth. Many monkey flower species are also pollinated by this moth.

Datura can stay open during the day, but is primarily a night-blooming plant. It is also called jimsonweed or locoweed. Photo: Charles Hood
Golden columbine seems to be part orchid, part party favor. Photo: Charles Hood
Finding a sphinx moth caterpillar tells us that spring has arrived in the desert. Photo: National Park Service
The white-lined sphinx moth hovers like a hummingbird and can be mistaken for one, as well as for a butterfly. Photo: Charles Hood

Moths and Bees

Moths come in many colors and shapes, and there are at least fourteen thousand species worldwide. Like daytime’s hummingbirds, moths come to night flowers for the nectar and only secondarily do the plant the service of moving pollen from plant to plant.

Small and overlooked as they may be, moths and other insects help make the landscape of the American West possible. For instance, Joshua trees are pollinated by yucca moths, which raise their young on the plant’s seeds; tree and insect are completely reliant on each other. Moths ignore political boundaries; migrating moths cross the border as freely as the air itself. Why are there so many moth-attracting plants in our dry Southwest? According to hawkmoth experts Robert Raguso and Mark Willis, “Moth pollination is more prevalent in the Southwest due to warm evenings, favorable climate, and proximity to the moth rich canyons and thornscrub of northern Mexico.”

The hawkmoths or sphinx moths (the names are interchangeable) are about four inches by four inches and famously have a foot-long tongue. That is a large insect, one easily watched with a pair of birding binoculars.

Other flower–insect interactions do not involve pollination. The desert globemallow is a sage-green plant with papery orange blossoms about the size of your pinky nail. The globe mallow bee does not return to a communal hive at night. Instead, it has a clever trick for sleeping. Just before sunset, the bee crawls inside a globemallow flower and waits. As the plant closes its blossoms for the night, the bee gets trapped inside and now has a warm, one-bee tent, inside of which it can rest until dawn.

A bee prepares to be "trapped" for the night inside a desert globe mallow flower. Photo: US Geological Survey

Queen of the Night

One night-flowering cactus blooms so rarely that when it does have flowers, it becomes the instant hit of the desert botanical season. Queen of the night cactus, also called the night-blooming cereus, is native to Texas, Arizona, and northern Mexico. It produces a fragrant white flower, usually in early summer. On average, the flower survives for only one night—if you hear that it is starting to bloom, drop everything and come see.

The queen of the night cactus blooms only one night a year. Photo: Fred Hood

The main stem looks like a dead thorn branch, at least above ground. Under the surface is the main energy reserve: a brown tuber shaped like a human tooth and as big around as a bowling ball. It can weigh up to fifteen pounds. All the queens in an area will bloom on the same night, hoping to attract hawkmoths. Later in summer, this cactus produces a red fruit, which is how the seeds are distributed for a new plant to start up. Few people pay attention to that—it is the blossom that gives the plant its fame.

The Tohono Chul Garden in Tucson maintains a collection of these cacti. A docent, Betty Carras, remembers what it was like in early days, before the bloom became famous. She says, “Watching carefully, we could actually see the opening spurts of the blossoms. Slowly but inexorably the bud became a flower. Ghostly white, it gradually opened its funnel-shaped blossom—becoming more aromatic as it increased in size. By the time its slender white petals were almost fully opened and the flower nearly five inches across, it was extremely fragrant. The velvet-like embrace of warm night air, the magic of a sweet-scented evening, and the distant chorus of singing coyotes made it truly a night to remember.”

Bats and Cacti, Cacti and Bats

Many borderland cacti and agaves use bats as pollinators. We will look at bats as a group starting on page 145, and while most bats worldwide use echolocation to hunt flying insects, many bats are nectar eaters or fruit gatherers, especially in the tropics.

Three nectar bats regularly come to the United States. Two are mostly in Arizona—the lesser long-nosed bat and the Mexican long-tongued bat—and one is mostly in Texas around Big Bend National Park, the greater long-nosed bat. Saguaros need Arizona’s bats; their pollination depends on it. These species also pollinate cardon and organ pipe cacti, as well as several agave species. They migrate here from as far away as Jalisco, and some have learned how to raid backyard hummingbird feeders. Since these bats can live up to twenty years, one yard may be being “robbed” by the same bat year after year.

A gibbous moon rises behind saguaro blossoms. Calling all bats! Photo: Fred Hood
Researchers caught this bat in a net. Its fur is yellow with pollen. Photo: José Gabriel Martínez-Fonseca

To slurp up nectar, a bat’s tongue has bumps along the side like a sponge or a pipe cleaner. All of this is quick as a flash: the tongue goes in and out of the flower (or hummingbird feeder) in about .40 of a second. The bat makes a swooping approach, but can’t really hover very well, so it brakes to a stall, takes a quick hit of sugar juice, and peels away like a jet leaving a dogfight. As they visit flowers and feeders, they can fly upright, sideways, and even upside down.

When not pollinating, bats still help, especially later in the season. They will eat the ripe saguaro tuna or fruit, distributing the cactus seeds via their guano. They are very good at this, in part because they cover a lot of ground; radio tracking shows nectar bats in the American deserts covering a hundred miles per night.

In and out—the raider grabs nectar at a feeder and leaves. Photo: Charles Hood

We know that hummingbirds are the classic nectar drinkers, yet all hummingbirds are also insectivorous. They cannot survive on sugar alone, so to create a balanced diet, they supplement flower visits with sallying after insects. By contrast, nectar bats’ snouts are long yet weak, and to save weight, these bats have given up most of their teeth. Flycatching the way a hummingbird does simply can’t happen. Instead, nectar bats use their long, flexible tongues to collect the pollen grains that accumulate on their fur, licking their way to good health. Hummingbirds can’t do this—their beaks and tongues don’t bend that way—nor can hummingbirds digest pollen. Nectar bats can (and do) digest pollen, and in fact, it is an important nutrient source for them.

For bats, a blooming saguaro does indeed provide “one-stop

A bat makes an approach to a test tube full of sugar water. Photo: José Gabriel Martínez-Fonseca
The tongue soaks up nectar almost too fast for the camera to catch. Photo: José Gabriel Martínez-Fonseca

Read The Night Garden

By: Katherine Renz

As we fall our clocks back an hour in November and the night creeps ever longer through December’s winter solstice, gardeners are presented with an opportunity to connect to the dark. After a nine-to-five workday and sundry obligations, it is often evening or later by the time one can make it into the garden. Introverts may thrive in the peace and stillness that descends once most of their loud fellow humans head indoors. In our relentlessly optimistic culture, it can be a relief for some of us to get in touch with our dark side, appreciating the free, subtle gifts of the twilight..

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