Biology concepts – nastic movements, turgor pressure, evolutionary pressure, tropism, osmosis
If you don’t let a Mimosa pudica (sensitive plant) plant rest at night, it will wilt away to nothing. A plant that needs a good night’s sleep? Really? We have talked about how sleep revitalizes different brain functions, especially within the hypothalamus (The Best Cure For Insomnia Is To Get A Lot Of Sleep), but plants don’t have a hypothalamus or any brain for that matter. So why does it die if it can't rest; is it out of its mind?
The prayer plant on the left is how it looks during the day, but
on the right, the leaves have folded or curled up. They also stand
straight up, as if at attention. A tough way to spend the night, but
it must serve some purpose.
The prayer plant (Maranta leukoneura) folds up its leaves at night and tilts them upward. When morning comes, the leaves tilt back into their day position and unfold to catch as much sunlight as possible. The folded leaves might look like they are praying (hence the name), and it may appear that they are sleeping, but this is just anthropomorphism.
Humans have a need to feel connected to the rest of Earth’s life, and in the process, we tend to see the behaviors of other organisms in human terms, trying to assign some human motivation to them. So, is the plant sleeping? Does it need to rest? No. Sleep in animals implies inactivity and neural rearrangement, and these don’t occur in plants.
However, the fact that the plant carries out this activity every night suggests that it has evolved in response to some pressure, some need. Surprisingly little is known about why plants move their leaves at night, but there are a few hypotheses. Some scientists believe that changing the angle of the leaves helps funnel dewdrops and overnight rain down the trunk or stem to the roots. Charles Darwin published two books on these plant movements, his theory being that the behavior reduced the chance of chill or freezing.
Another hypothesis suggests that leaves fold up to keep the rain from pooling on them and promoting bacterial or fungal growth. Or perhaps, apposing one leaf closely to the opposite leaf reduces the amount of water lost overnight. However, aquatic plants don’t have to worry about loss of water, but some immersed plants, like Myriophyllum Mattogrossense, still fold up at night. It may be a holdover from their terrestrial days, as most of today’s aquatic plants evolved from terrestrial plants.
My personal favorite proposes that by folding up their leaves, the plants give nocturnal predators a better shot at seeing, hearing, and smelling nocturnal prey. By helping the predators, plants are indirectly protecting themselves from animals that would eat them- plants are sly little devils (more anthropomorphism). It is probable that different plants move for different reasons, so one hypothesis almost certainly won’t cut it for all organisms.
Plants have night moves other than folding leaves. Morning glories (Ipomoea violacea) close their flowers overnight. The reasons for this movement may be a little plainer. Dry pollen sticks to pollinators better than wet pollen, so closing off the stigma to rain or dew keeps the pollen dry. It also takes energy to maintain an open flower; this energy would be best spent when pollinators are around. If the plant’s pollinators are diurnal, they why leave the buffet open all night?
Just as animals have an internal clock, plants gauge
their movements according to the circadian period.
Often plants match their rhythms to pollinator animals
they depend on or to avoid the active periods of
predators. Anyway, I like the picture.
There are also flowers that have the exact opposite behavior, opening their flowers as the sun sets. Philodendron selloum (Is It Hot In Here Or Is It Just My Philodendron) is a classic example, with its spathe closing down in the early morning hours.
Moonflowers (Ipomoea alba) are another example. At about 8:00 pm, the moonflower opens. A single flower can go from completely closed to fully open in less than a minute (http://www.moonlightsys.com/themoon/flower.html). The morning glories and the moonflowers are both of genus Ipomoea, but they have opposite behaviors – different pressures lead to different adaptations, even in closely related species.
These movements of plant structures are independent of the direction of the stimulus, ie. they are not following the sun or being blown by a particularly wind, so they are called nastic movements. Nyctinasty (nyc = night or darkness, nastic = firm or pressed close) is the specific movement of leaves or flowers in a daily pattern, open during the day and closed at night. If directed by the position of a stimulus, the movements are called tropisms (heliotropism, thigmotropism, gravitropism).
Nyctinastic movements are accomplished by the flow of water in and out of specific cells in the pulvini (swellings, singular is pulvinus) at the base of the petioles (the stalk that attaches the leaf blade to the stem). It is not unlike our muscle movements in that there is an extensor and a flexor pair. When K+ and Cl- are pumped into the extensor cells on the bottom of the pulvini, they become hypertonic and water follows the ions through osmosis. This causes the extensor cells to swell due to increased turgor pressure.
Turgor pressure refers to the pressure of the cell contents against the cell wall. This increased turgor pressure at the bottom of the petiole pushes the leaf up. In an opposite fashion, night causes a movement of ions to the flexor cells on the top of the petiole. Water flows out of the extensors and into the flexors by osmosis, causing the stem to droop. Flowers and leaves open and close by the same movements, with the extensor and flexor cells located at their bases.
Turgor pressure is the same mechanism which causes the venus flytrap (Dionaea muscipula) to snap closed its jaws of death when an insect disturbs its trigger hairs. These hairs are located on the nectar laden, red lobes of the trap. Touching just one trigger hair doesn’t spring the trap, two must be displaced within 20 seconds of each other. This saves energy and unnecessary trap closings; each trap snaps shut only four or five times, then dies. If you thought the moonflower moved fast, check out the venus flytrap (). I’m just surprised we can’t hear the water shooting into the flexor cells!
The trigger hairs are mechanosensors. The stimulus that trips the trigger and causes the flow of ions and water in the extensor and flexor cells of the hinge region is directionally irrelevant; therefore, the snapping shut of the trap can be considered a nastic movement. In this case, as with the sensitive plant (Mimosa pudica), the movement is called haptonasty (hapto = touch).
A small percentage of plants have nyctinastic movements, so they are an exception to the rule that plants don’t move actively, but even a small percentage means that thousands of species do have these movements. This many exceptions underscores the point that nyctinasty must perform an important function.
Just as humans with fatal familial insomnia die from a lack of sleep (An Infectious, Genetic Disease), the sensitive plant has a much shorter lifespan when nyctinasty is prevented. A plant hormone that stimulates leaf opening was identified in 2006. When given to plants continuously, it caused the leaves to remain open. When nyctinasty was prevented in this way, the leaves were noticeably damaged within a few days, and the plant was dead in less than two weeks. It may not be sleep, but whatever it is, it is just as important.
Some plants are open during the day and some are open at night, just as some animals are active during the day and some during the night. And just as plants adapt to a time schedule to promote survival, animal adaptations are crucial to life in the light or the dark. But that doesn’t mean that some organisms won’t throw us a curve, as we will discover next time.
Ueda, M. and Y. Nakamura. 2006. Metabolites involved in plant movement and ‘memory’: nyctinasty of legumes and trap movement in the Venus flytrap. Natural Product Reports, 23 (4): 548-557.
For more information, classroom activities or laboratories on nastic movements or turgor pressure, see:
nastic movements –
turgor pressure –