In the scenario of sealing a house plant in a transparent glass container, the primary factor that would lead to the plant's death over time is a lack of carbon dioxide.
Plants require carbon dioxide for photosynthesis, which is the process they use to convert light energy into chemical energy. In a sealed environment, the plant may initially have enough carbon dioxide for photosynthesis, but as it continues to use the carbon dioxide and produce oxygen, the levels of carbon dioxide can become insufficient. This could eventually halt photosynthesis, leading to the plant's death.
Although a lack of light could also contribute to the plant's demise, especially if it is not receiving adequate light for photosynthesis, without carbon dioxide, the plant cannot perform photosynthesis effectively even if there is light available. Therefore, a lack of carbon dioxide is the critical factor in this scenario.