无性生殖与有性生殖的主要区别在于（   ）。

阅读下文，完成各题。

土地

大地

                                                                                           亿万年的静止，此刻在眼里

                                                                                           连绵起伏，波涛汹涌

                                                                                           祖辈们和树木花草

                                                                                           一起扎根

                                                                                           草木春茂冬枯

                                                                                           人在一代接一代

                                                                                           繁衍

                                                                                           父母，是一段

                                                                                           枯老的树干

                                                                                           我就是那横出的新枝

                                                                                           站在父母的肩上

                                                                                           以向上的姿势飞翔

                                                                                           风，从远古吹来

                                                                                           我用嫩叶为帆远航

                                                                                           漂得再远，根都在父母身上

                                                                                           土地的深处

当前我国主要的生态环境问题是(     )