The Flora of Iran is so diverse and it is one of the richest plant diversity countries in Southwest Asia. The great difference between the two latitudes of the north and south of the country and the existence of different plains, altitudes, and climates have given a very diverse view to the vast land so that a variety of vegetation and plant species are grown across it (Nasab and Khosravi, 2020).

Endemic plants are present worldwide and determine the identity of local flora (Stebbins 1980). Climate, ecology and geology are among the critical factors that determine species' distribution (Kruckeberg and Rabinowitz, 1985; Stace, 1993, Debussche and Thompson, 2003; Mirek and Pie˛koś-Mirkowa, 2009). Based on age, distribution, or cytogenetic characteristics, various types of endemic classification have been shown (Kruckeberg and Rabinowitz, 1985). Endemism-based research usually focuses on aspects of ecological differentiation or adaptation to newly developed habitats (McKay et al., 2001, Debussche and Thompson, 2003, Rajakaruna, 2004).

Gypsum soils are one of the most widespread and unusual substrates in the world (Kayabas, 2020). They are present on all continents and host a highly diverse and specialized flora that has been designated as a conservation priority (Kayabas, 2022). The gypsum habitats of Iran are significant reserves of biodiversity containing endemic and rare species. Plants in gypsum habitats are very diverse, but only some of them are endemic. These are called gypsum species and grow almost exclusively on gypsum soils (Bolukbasi et al., 2016). According to the biosystematics of gypsum plants, the range of distribution of exclusive plants is determined by ecological factors including soil and altitude. Species that grow exclusively in gypsum soils are referred to as exclusively gypsophyte species. In gypsum habitats, a wide variety of plants live, but some of them are uniquely adapted to the climatic and soil conditions (Kruckeberg and Rabinowitz, 1985). Meyer (1986) investigated the occurrence of exclusive plant species according to soil type, and found that gypsum plants (gypsophytes) are found in gypsum soil. He divided gypsophytes into two groups: gypsophiles, which grow only on gypsum soils, and gypsovages, which grow on gypsum as well as non-gypsum soils). Palacio et al. (2022) later recognized this classification, which includes various rare plant species found in arid and semi-arid regions all over the world.

There is a strong association between the endemic flora of gypsum habitats and mineral gypsum materials in dry regions of the world (Escudero et al., 2014). These species can only be found in soils that are saline, limestone, serpentine or gypsum. Crystals are prevalent in gypsophytes plants, and organic molecules accumulate sulfates. These plants absorb calcium and sulfur from calcium sulfate-containing soils through a specialized mechanism in their roots. (Cera et al., 2022). The expansion of gypsum habitats at the global level increases the importance of their biological protection (Cañadas et al., 2014). Seven Semnan-endemic species and 21 Iran-native species grow in the gypsum habitat of Semnan. As a result of the harsh ecological conditions in Semnan, low rainfall, and high summer temperatures, these plants have been able to adapt to their environment. Calcium sulfate in this type of soil has caused the preservation and survival of plant species.

The close relationship between plants and specific geological substrates has long been studied (Kruckeberg, 1986). Habitats with gypsum soils, which have significant geobotanical effects such as drought, nutrient limitation, and ion toxicity, are a stressful environment for plants growing on them (Liaquat et al., 2020, Liaquat et al., 2021). In gypsum habitats, there is an imbalanced concentration of nutrients in the soil. Especially, excess calcium (Ca), sulfur (S) and magnesium (Mg) caused by gypsum causes a lack of essential nutrients such as nitrogen and phosphorus (Bolukbasi et al., 2016). In order to deal with the excessive concentration of these elements in the soil, plants must find mechanisms to regulate the concentration of intracellular calcium, sulfur and magnesium. Such mechanisms are the basis of plant adaptation to gypsum soils. Although plant endemism can be associated with genetic variation, it is also often associated with morphological, anatomical, and ecological traits. The species that grow in gypsum soils have different morphological characteristics (Kayabaş and Mutlu, 2022; Kayabas, 2021a; Kayabas, 2021b). Precipitation of calcium oxalate, common in a large number of plant species, is a controlled consequence of cell differentiation in plant cells (Kausch and Horner, 1983). Clusters of needle-shaped crystals, called raphides, are produced inside cells called crystal idioblasts (Foster, 1956). Idioblast crystal initials usually form in metastatic tissues and differentiate prematurely compared to surrounding cells (Kausch and Horner 1983). The role of plant calcium oxalate crystals in normal tissue growth and function is still not fully understood. Crystal formation occurs either within cytoplasmic stands or extracellularly within plasma membranes, such that crystals appear to be located within central vacuoles. The presence of hairs on the plant helps remove excess gypsum from the plant in the form of crystals on its surface. This ensures the plant's adaptability to strong evaporation of water. The sessile glandular hairs in the leaves of gypsum species can be examples of morpho-anatomical adaptations. Stomatal architecture varies widely among plants, and some features are considered anatomical adaptations. Xeromorphic properties decrease the conductance of stomata situated in crypts (Sundberg, 1986). This paper examines the characteristics that contribute to the adaptability of gypsum plants and enable them to survive in extremely dry and arid areas. To adapt to drought in xeric and gypsum regions, these plants can adapt their leaf structure to anatomical adaptations.

Micromorphological studies provide valuable diagnostic characters for distinguishing many species (Amini et al., 2011, Amini et al., 2019a). Considering the relative stability of some anatomical traits related to environmental factors, anatomical results are critical in species systematics (Keshavarzi et al., 2013). The classification value of epidermal anatomical features is well documented in the botanical literature (Amini et al., 2019b). So far, no study has been conducted on the relationship between the endemic species of the gypsum soils of Semnan and environmental factors. Kayabas (2022) studied the SEM observations of Acantholimon riyatguelii Yıldırım (Plumbaginaceae), endemic to gypsum habitats in Turkey. A review paper published by Escudero et al., 2014 examined plant life on gypsum. They stated that gypsum habitats increase our knowledge of limiting soils and provide ideal models for improving community ecosystem performance. Rabizadeh et al. (2019) investigated the ecological-anatomical comparative adaptability of two gypsophylic Astragalus species in gypsum soils. Rabizadeh (2020a) studied the anatomy, morphology and ecology of the endemic Iranian Moltkia gypsacea from the Boraginaceae family. Rabizadeh (2020b) reported the ecological, anatomical, morphological, and micromorphological characteristics of Gypsophila mucronifolia (Caryophyllaceae) endemic to gypsic soils of Iran.

We have investigated ultrastructures in the vegetative parts of a local endemic species restricted to gypsum habitats in Semnan. In this study, we examined how these plants adapt to dry conditions and gypsum soils.