April 28, 2025

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Today I would like to highlight a rather unique yet unassuming native plant in our butterfly garden. It’s one you might overlook due to its low profile as a ground cover, the Mimosa strigillosa or Sunshine Mimosa. But, first a little plant biology to help you understand the unique quality of our Sunshine Mimosa.

Ever since Charles Darwin published “The Power of Movement in Plants” in 1880, scientists have been studying plant movement. Besides the act of growing, do plants move? How and why might plants move? What types of stimuli might contribute to plant movement?

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There are two types of plant movement, tropic movement and nastic movement.

 

Tropic Movement is movement toward or away from a stimulus, it is directional. Phototropism enables plants to optimize sunlight for photosynthesis. Young sunflowers are able to track the sun’s movement throughout the day to gather sunlight in a crowd of wildflowers by turning their flower heads to follow the sun. Another example of a tropic movement is hydrotropism. Roots will grow toward the direction of a water source. Tropic movements are movements enabling plant growth.

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Nastic Movement is also a response to stimuli but it’s nondirectional, repeatable, and not a growth response. Do some plants sleep?  You might say that. There are plants that respond to a lack of intensity of light, will close their petals or leaves at night seeming to sleep, and with increased intensity of light, open back up in the morning. You have probably seen tulips and morning glories do this.  Sunshine Mimosa also sleeps, folding its leaves at night and waking in the daylight. Why do plants sleep ? Some may do so to protect their pollen or others to reduce water loss while their leaves are notphotosynthesizing. This conservation of water may be particularly beneficial in times of drought.

 

Another fascinating nastic movement is a plant’s response to touch. The Venus Flytrap, a carnivorous plant, exhibits this nastic movement when a trap shuts when touched by an insect. Florida’s Sunshine Mimosa, or Sensitive Plant, also responds to touch. The name Mimosa is thought to be Greek meaning to mimic, as the plant mimics animal behavior. Our plant can move, although without the use of muscles. If something touches a Sunshine Mimosa leaf, the leaf will fold up. To do this, an electrical signal moves quickly to the area at the base of the leaflet where it meets the stalk, known as the pulvinus. There specialized motor cells, extensor cells and flexor cells, move water between them causing the needed pressure to fold the leaflets. This action is repeatable and reversible, the leaflets will open back up again maybe in seconds or maybe minutes. Then they might close again. It’s interesting that the speed of the response depends on the magnitude of the stimulus. Hitting the leaf hard with the flick of a finger will close a leaflet in the blink of an eye, whereas a gentle touch of a child will result in a slower closing.

 

This sensitivity to touch may be a defense against browsing herbivores trampling over this ground cover plant, who might be startled by the movement and elect not to eat the plant. Or an animal might move on as the leaves when folded look wilted and not tasty. Also, the leaf movement itself may be enough to throw off small insects thereby protecting the plant.

 

Our pretty little sensitive plant can be found in the wild in southeast US and in almost every county in Florida. It is now in bloom, its pink pompom flowers on top of 4 to 8 inch bare stalks. Each pompom is actually many little flowers bundled together, the globe shape formed by many stamens.

 

When children visit our garden, there are interesting things to investigate together. After talking about the butterfly life cycle and hunting for butterflies with children, it’s fun to show children our sensitive plant. Let them stroke the leaves and think about why a plant would want to fold up its leaves. Have them try other plants to see that not all plants respond to touch. Ask what they think a Sunshine Mimosa flower looks like? Make up some new nicknames for our Sunshine Mimosa, which besides being

called the sensitive plant, is also called touch-me-not and tickle me plant.

I invite you all to go out to the garden now and discover nature through the

eyes of your inner child. Enjoy!