② The main metabolites of plants are carbon dioxide, water and protein, as well as nitrogen-containing wastes produced by nucleic acid metabolism, such as urea and ammonia. Before the leaves of plants fall off, on the one hand, the reusable things and nutrients in the leaves are transferred away, on the other hand, some metabolic wastes are transferred to the fallen leaves. So the fallen leaves of plants can take away some waste.
③ Spirogyra, which lives in water and has no differentiation of roots and leaves, belongs to algae, while bryophytes have differentiation of stems and leaves;
(4) Coniferous forest is the general name of all kinds of forests with conifers as the main tree species, including evergreen deciduous, cold-resistant, drought-tolerant, warm-loving, wet-loving and other types of coniferous pure forests and mixed forests. It is mainly composed of some cold-resistant tree species such as spruce, fir, larch and pine. Therefore, coniferous forest vegetation types are mainly pine and fir.
⑤ The fruit consists of seed and pericarp. Watermelon is a fruit, and the seed coat and peel of wheat are combined, so the grain of wheat is a fruit, not a seed.
6. Water is the main component of plants. Water is the substance for plants to carry out various physiological activities, such as RJI agent for chemical reaction, material transportation, raw materials for photosynthesis and so on. Because various plants have different needs for water, the water in the biosphere affects the distribution of green plants. For example, the temperature in Xilin Gol is similar to that in Changbai Mountain. Vegetation types are different because Changbai Mountain has more rainfall than Xilin Gol, and there is enough water for plants to grow, so Changbai Mountain can grow dense forests.
Therefore, choose: B.