###### Abstract  
In recent years, new sources of secondary metabolites (SMs) in medicinal plants have been identified, and the introduction of these plants into field conditions has been carried out to obtain chemically diverse standardized raw material (RM). An example is mountain arnica montana L, one of Europe's endemic endangered medicinal plant species, commonly used in pharmacy, cosmetics, and medicine. Its infloreserences (_Arnica flos_) are characterized by anti-inflammatory, antiradical, antioxidant, antibacterial, antifungal, and antitumor properties. The main goals of the present research included: (i) characterization of the chemical composition of the infloreserences of _A. montan_ harvested in different development stages; and (ii) presentation of the role of the development stage and different extraction methods in the antitumor activity of extracts through analyses of apoptosis, autophagy, and necrosis induction in human cervical carcinoma HeLa, human colon carcinoma HT29, and human colon metastatic carcinoma SW620 cell lines. The development stage was found to modify the composition of pharmacologically active substances, e.g., sesquiterpene lactones (SLs), flavonoids (Fs), and essential oil (EO), in amica infloreserences. The content of Fs and EO increased during flowering to the full flowering phase; however, the highest content of SLs was noted in the full flowering phase and at the end of flowering. More promising results, i.e., a relatively high level of apoptosis and a low level of necrosis induced by the amica extracts, were demonstrated in the HeLa cell line (full flowering; concentration: 0.5 \(\mu\)L/mL), the HT29 cell line (beginning of flowering; concentration: 0.5 \(\mu\)L/mL), and the SW620 cell line (stage of yellow buds; concentration: 1 \(\mu\)L/mL). This extremely valuable medicinal plant species provides a very broad range of RMs (e.g., infloreserences, rhizomes, roots, achenes, and all plant); therefore, attention should be paid to the more frequent use of water as a solvent in studies on the biological activity of mountain arnica extracts. Arnica montana; development stage; extraction methods; anticancer activity +
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Footnote †: email: pauduch@mail. been carried out to obtain standardized raw material (RM) [7; 8; 9]. An example is mountain arnica _Arnica montana_ L. from the Asteraceae family, which has recently gained great levels of interest from researchers [10; 11; 12]. It is worth noting that mountain arnica, which has been considered a valuable source of RM for the industry for many decades, is also a protected species in Europe, and its populations are endangered [13]. _A. montana_ is an endemic mountain plant species in Europe [14; 15; 16]. Therefore, the introduction of this plant to field cultivation primarily reduces the pressure of the acquisition of RM from natural habitats, and, secondly, facilitates the application of cultivation factors that contribute to the generation of high yields of active substances and the production of more chemically diverse herbal RM [7; 8; 9]. The yield of SMs in the RM of cultivated medicinal plants, their composition, and their chemical differentiation depend on factors such as soil quality [7; 9], soil and foliar fertilization [3; 6; 17; 18], seasonal variations [19; 20], plant density [2], and weather conditions [21; 22]. In addition to environmental factors that determine the chemical profiles of medicinal plants, other important factors include the propagule type during introduction [9], the stage of development [19], morphological features, and plant organs [23; 24]. In this way, the quantity and quality of the RM can be modified based on the knowledge of the biology and ecology of the species and with the use of agricultural tools.