世代交替

植物的生活史中，二倍体的孢子体世代（又称无性世代）和单倍体的配子体世代（又称有性世代）有规律地相互交替的现象.植物界中的部分绿藻、绝大多数红藻和褐藻、全部高等植物（包括苔藓、蕨类、裸子和被子植物）的生活史均具有世代交替.世代交替又可分为两种基本类型，一为同形世代交替，即孢子体和配子体在形态大小上类似，仅在细胞学上和生理上有所不同，如石莼等；另一类型为异形世代交替，即孢子体和配子体在形态结构和大小上也有明显差异.其中又分为孢子体发达的异形世代交替和配子体发达的异形世代交替.前者为孢子体大型，结构复杂，生活时期长，配子体弱小，生活时期短，如海带等藻类植物和蕨类、种子植物；后者相反，为配子体发达，体较大，孢子体弱小，如紫菜等，或不能独立生活而寄生在配子体上，如苔藓植物.Life cycle of plants, the diploid sporophyte generation (also known as asexual generation) and the haploid gametophyte generation (also known as sexual generation) are regularly interspersed phenomenon. Plantae in the part of the green algae, must Most red algae and brown algae, all higher plants (including mosses, ferns, naked child, and angiosperms) and all have alternation of generations life cycle. alternation of generations can be divided into two basic types, one for the same shape alternation of generations, the sporophyte and the gametophyte in the morphology similar size, only cytology and physiology are different, such as Ulva, etc.; another type of shape alternation of generations, that is, in the sporophyte and gametophyte morphology and size are also significant differences. which are divided into sporophyte developed special-shaped generational and generational shaped gametophytes developed. The former sporophyte large, complex structure, life during the long, frail small gametes, living a shorter period, such as kelp and other algae, plants and ferns, seed plants; the latter the contrary, for the gametophyte developed , body larger spores frail small, such as seaweed and so on, or inability to live independently and parasites in the gametophyte, such as bryophytes.

特征

　　植物中世代交替以蕨类植物比较明显，孢子体和配子体都能独立生活。二倍体的孢子体进行无性生殖时，孢子母细胞经过减数分裂产生单倍体（n）的孢子，孢子萌发长成小型的

能独立生活的配子体，叫做原叶体。原叶体在进行有性生殖时，分化出雌雄器官，即颈卵器与精子器，并分别产生卵和精子。这两种配子配合形成了二倍体（2n）的合子。合子又长成下一代新的孢子体。世代交替在各类植物中，因孢子体与配子体的形态、大小、显著性、生活期限以及能否独立生活等方面的不同，差别很大，但基本过程与蕨类植物是一致的。