How water pressure replaces muscle contraction
In animals, muscles pull on bones by contracting. Plants lack muscles, but they have a comparable driving force: turgor pressure, the internal hydrostatic pressure of water inside plant cells [1][4]. When a plant cell fills with water, it pushes outward against its stiff cell wall, and when it loses water, the pressure drops. By controlling the flow of water and ions into and out of cells, plants can make tissues swell or shrink in a coordinated way, producing movement without any contractile fibers [1].
The best-studied example is the opening and closing of stomata, the tiny pores on leaves that regulate gas exchange. Stomatal movement is driven by turgor pressure changes in the guard cells that surround each pore, and many of the genes responsible for ion transport during this process have been identified [1]. The same basic mechanism—water-driven swelling and shrinking—also operates in the pulvini, the motor organs at the base of leaves that produce daily sleep movements in many plants [1].
How some plants achieve rapid motion despite the limits of water transport
Moving water through plant tissues is relatively slow, and this imposes a limit on how fast a plant part can change shape: the larger the moving part, the more severe the limit [1]. For slow movements like stomatal opening or daily leaf adjustments, this is not a problem. But some plants, such as carnivorous traps, move very rapidly—at speeds comparable to those of animals—and they overcome the water-transport limit by using the mechanical behavior of their three-dimensional structures [1]. In these cases, elastic forces stored in the tissue are released suddenly, much like a snapped branch or a released spring, rather than relying solely on the speed of water movement [1].
Rapid multicellular movements also require a way to coordinate cells quickly. Electrical activities similar to animal action potentials have been found in many plant species and are promising candidates for the rapid cell-to-cell signaling behind fast movements, though the molecular identity of these electrical signals remains obscure [1]. This means that while the involvement of electrical signaling is suggested, it is not yet fully understood.
Hormones and internal clocks set the timing of plant movements
Many plant movements are not just reflexive responses to immediate stimuli but are tied to internal daily rhythms. A class of plant hormones called turgorins has been identified as endogenous regulators of daily rhythms in higher plants, controlling movements that occur without muscles [4]. These hormones help plants anticipate regular changes in their environment, such as the alternation of day and night, and adjust their movements accordingly.
The temporal dimension of plant movement is an active area of study. Research using kinematical analysis is beginning to describe how plants structure their movements in time, from growth and adaptation rates to behavioral strategies and communication [3]. This work raises questions about how plants perceive the passage of time and coordinate physiological processes, but it does not yet provide a detailed mechanism for how muscles are bypassed [3].
From early ideas about plant nerves to human-made plant-like muscles
The idea that plants might have something analogous to a nervous system is not new. As early as 1926, Bose examined plant responses and reactions that occur without muscular involvement, exploring how plants might coordinate movement and sensitivity in the absence of muscles [2]. While the molecular details were unknown at the time, this early work framed the question of how plants achieve coordinated responses without animal-like nerves or muscles.
Modern engineering has taken inspiration from plant movement. Researchers have created artificial muscles made of hierarchically patterned, helically wound yarns that mimic the hydrotropism and thermotropism of helical plants—that is, their ability to respond to moisture and temperature by moving [5]. These synthetic actuators replicate plant-like microstructural memories at the microfibril level and can autonomously close a window in wet climates, demonstrating that the principles of plant movement can be harnessed in materials that contain no biological muscle at all [5].
Sources used in this answer
Rapid movements in plants
Rapid plant movements often use turgor pressure and elastic forces rather than muscles, with stomatal movement as a well-characterized model and electrical signals as candidate rapid cell-cell communication.
The nervous mechanism of plants
Early work examined plant responses and reactions that occur without muscular involvement, suggesting plant nervous mechanisms.
Temporal Dynamics With and Without a Nervous System: Plant Physiology, Communication, and Movement
The review discusses temporal dynamics of plant movement and communication, using kinematical analysis, but does not detail how plants move without muscles.
Turgorins, hormones of the endogeneous daily rhythms of higher organized plants—detection, isolation, structure, synthesis, and activity
Plants move through changes in turgor pressure rather than muscle contraction, with turgorins acting as hormones regulating endogenous daily rhythms.
Plant‐Like Tropisms in Artificial Muscles
Artificial muscles made of helically wound yarns mimic plant tropisms and achieve rapid movement without biological muscles, demonstrating plant-like actuation principles.