John and George searched Jonathan Dickinson State Park today for “Cut Throat Grass” we didn’t find. Rough walking and a thunderstorm kept camera gear in the car. Even if we missed the bloody grass there’s always plenty to see, and today under exceptionally dark skies it was enlightening to study the leaf positions on the various Legume weeds, such as Sensitive Brier, Pigeon Pea, Milkpeas, Amorpha, and Rabbit Bells.
Many plants move their leaves upward or downward at different times for different reasons, mostly up and perky in the bright of day, and collapsed and droopy at night, sometimes in-between during shade or “down” under stressful conditions. Most notably, many go to sleep at night and wake up in the morning.
Foliar “sleep movements” have fascinated biologists for about as long as there have been biologists. Charles Darwin himself tackled the topic. Perhaps he just liked to say, “nyctinasty,” the fancy term for what we’re discussing. As Bob Seeger might tell you, it means “night moves,” a good name for a band:
Plants with moving leaves include Oxalis, Velvetleaf, members of the Maranta Family, and especially most Legumes. Ever notice how Legume trees such as Royal Poinciana can sleep at night, or how veggies such as beans and peanuts, take a flop, or how backyard weeds, such as Tick-Trefoils collapse, or how native Legume species know when to fold em’?
The motion comes mostly from changes in cellular water pressure. Legumes have hydraulic leaf-lifts called pulvini (singular pulvinus) visible at the base of the compound leaf where it joins the stem. The pulvinus is the thick little muscle immediately adjacent to the attachment point. Sometimes leaflets within a compound leaf have their own little pulvini.
Initiating the motion is where it gets tricky. What pushes the “lift the leaf” button? Environmental cues come mostly from light and dark, specifically from red and blue light. Usually even more important is an internal clock. A built-in rhythm. For a little taste of competing control by light and by a built-in clock, enjoy a little time lapse movie:
The internal clock needs dawn and dusk for setting. If you leave a plant in constant light for enough days the up-down cycle continues but degrades in timing and in intensity until it fizzles out.
Next time you are out and abut among trees at night, take a look at the Legumes: Royal Poinciana, Pride of Barbados, Albizia, Powderpuffs, Sennas, Princess Earring, Pongam, Copperpod, and others, do they sleep at night?
This diagram shows deterioration of the leaf response cycle in continuous light. The vertical axis shows the degree of erectness of the leaves in terms of percent. The horizontal axis shows 5 pm through the night to 10 pm the next day. By placing three lines representing three nights) on the graph, three days of response by one Tick-Trefoil plant are condensed into one graph.
The blue line (Day-night cycle) shows leaf movements in natural day-night light-dark. The leaves dropped abruptly and fully 6-7 pm, then stayed fully down until rising abruptly and fully 7-8 pm.
The orange line (24 hours constant light) shows the same plant placed in constant light for a night after a night under natural conditions. It drooped full but required two hours 7-9 pm, and started waking up 2-3 AM and required about 4 hours to become almost 100% erect. Its response was mildly degraded.
Continued under constant light through the following night shown by the gray line, the same plant (48 hours constant light) drooped gradually and incompletely, hitting its lowest point around 4 AM, then rising slowly and incompletely until the experiment ended at 10 AM. Its response was severely degraded.