植物呼吸作用的实质是（    ）

                                                                                                    山居秋暝
                                                                                                      王　维
                                                                                        空山新雨后，天气晚来秋。
                                                                                        明月松间照，清泉石上流。
                                                                                        竹喧归浣女，莲动下渔舟。
                                                                                        随意春芳歇，王孙自可留。