# Exploring the Mechanisms of Humidity Responsiveness in Plants and Their Potential Applications
## 1 Introduction  
Plants have excellent sensory systems, behaviors, adaptations, and cognitive functions [1; 2; 3] and are highly sensitive to the mutual network between the environment and plant body. Hyroscopic behavior is defined as the phenomenon wherein water molecules are drawn in from the surrounding environment through absorption. The main factors that contribute to the hygroscopic movement in plants include the driving force of turgor pressure, the mechanism of swelling-driven deformation, and the trajectory of movement. As plants that exhibit hygroscopic behavior show atypical external mechanisms due to internal structural changes, studying their structural aspects from both a biological and ecological standpoint is valuable. Although the various angles of hygroscopic behavior of plants have been researched [4; 5; 6] in order to apply biomimicry in engineering technology [7; 8], there is limited knowledge about the adaptive anatomy related to the altered shape, structure, and physiology of plants [9]. It is widely known that plants use their own unique materials and structures to adapt to and survive in their surroundings. In other words, living things in nature have the characteristic of responding to external environmental stimulifor environmental sensing, food search, and reproduction. With recent rapid industrial growth, environmental issues have come to the forefront, emphasizing the importance of environmental sensing technology. Biomimetics refers to the advancement in sustainable technology developed by applying the basic principles and structures of biological and ecological resources. It is a field of application that aims to solve social problems by mimicking nature's unique characteristics and is a new-growth power in the Fourth Industrial Revolution [10; 11]. Biomimetic research is focused on approaches from the engineering field for solving technical problems [12; 13]. However, acquiring the sources of biological and ecological knowledge needed for technology development remains challenging [14; 15]. The genera _Selaginella_, _Erotium cicutarium_, and _Pinus_ are examples of humidity-sensing plants, and the related studies mainly focus on engineering theory [16; 17; 18]. In particular, the mechanisms of xerophytes, hygophytes, and semi-hygrophytes are distinct [19; 20], but hygroscopic movement in response to moisture, as mentioned in this study, is considered a common function. In _Selaginella involveus_, the leaves curl toward the central axis (stem) in dry conditions and uncurl under sufficient relative humidity [21]. In _Pyrrosia lingua_, a perennial evergreen herb that grows on tree stems or rock surfaces, a similar phenomenon is observed. To understand the functional and behavioral mechanisms underlying the hygroscopic behavior of plants, research on plant species is necessary to select target species allowing biomimetic materialization and to appreciate their structural specifications. Consequently, there is a growing trend in demand for environmental sensing sensors designed to detect prominent environmental factors such as humidity, temperature, air pollution, and harmful gases. Especially humidity has direct and indirect impacts on everyday life and industrial environments. Technologies are being developed, leveraging the principles of plant humidity detection and the integration of engineering techniques. These include temperature-responsive sensors based on eco-friendly materials, enabling information transmission without the need for external measuring equipment or power sources. In this study, the morphological structure and function of the genus _Selaginella_ and _Pyrrosia lingua_ were analyzed to understand the intricate deformation of internal structure caused by humidity changes, with the goal of developing technological applications through biomimetics. Finite element analysis was performed to appreciate the deforming tendency caused by hygroscopic swelling.