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The majority of dry mass in plants comes from CO2 assimilated during photosynthesis. This process involves the conversion of carbon dioxide from the atmosphere into organic compounds, primarily glucose, through a series of reactions that utilize sunlight as energy. During photosynthesis, plants take in carbon dioxide and water, using light energy to convert these into glucose and oxygen. The glucose serves as the building block for various biomolecules, including cellulose, starch, and other carbohydrates, which contribute to the plant's structure and energy storage.

Over time, the carbon acquired from CO2 forms the bulk of the plant's biomass, hence driving the growth and development of the plant. While inorganic compounds from the soil do provide essential nutrients and minerals, they do not contribute significantly to the dry mass compared to the carbon dioxide assimilated during photosynthesis. Similarly, while water is crucial for many physiological processes, it does not contribute to dry mass because it evaporates and does not contribute to the solid content of the plant. Oxygen produced during respiration is a byproduct of metabolic processes and does not play a role in building the plant's biomass.

Therefore, the assimilation of CO2 during photosynthesis is the primary source of dry mass in plants, highlighting the importance of this process in plant growth