<P> Eduard Strasburger (1874) discovered the alternation between diploid and haploid nuclear phases, also called cytological alternation of nuclear phases . Although most often coinciding, morphological alternation and nuclear phases alternation are sometimes independent of one another, e.g., in many red algae, the same nuclear phase may correspond to two diverse morphological generations . In some ferns which lost sexual reproduction, there is no change in nuclear phase, but the alternation of generations is maintained . </P> <P> The diagram above shows the fundamental elements of the alternation of generations in plants . The many variations found in different groups of plants are described by use of these concepts later in the article . Starting from the right of the diagram, the processes involved are as follows: </P> <Ul> <Li> Two single - celled haploid gametes, each containing n unpaired chromosomes, fuse to form a single - celled diploid zygote, which now contains n pairs of chromosomes, i.e. 2n chromosomes in total . </Li> <Li> The single - celled diploid zygote germinates, dividing by the normal process (mitosis), which maintains the number of chromosomes at 2n . The result is a multi-cellular diploid organism, called the sporophyte (because at maturity it produces spores). </Li> <Li> When it reaches maturity, the sporophyte produces one or more sporangia (singular: sporangium) which are the organs that produce diploid spore mother cells (sporocytes). These divide by a special process (meiosis) that reduces the number of chromosomes by a half . This initially results in four single - celled haploid spores, each containing n unpaired chromosomes . </Li> <Li> The single - celled haploid spore germinates, dividing by the normal process (mitosis), which maintains the number of chromosomes at n . The result is a multi-cellular haploid organism, called the gametophyte (because it produces gametes at maturity). </Li> <Li> When it reaches maturity, the gametophyte produces one or more gametangia (singular: gametangium) which are the organs that produce haploid gametes . At least one kind of gamete possesses some mechanism for reaching another gamete in order to fuse with it . </Li> </Ul> <Li> Two single - celled haploid gametes, each containing n unpaired chromosomes, fuse to form a single - celled diploid zygote, which now contains n pairs of chromosomes, i.e. 2n chromosomes in total . </Li>

In a paragraph describe alternation of generations in ferns