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All land plants, and some algae, have life cycles in which a haploid [[gametophyte]] generation alternates with a [[diploid]] '''sporophyte''', the generation of a [[plant]] or [[alga]] that has a double set of [[chromosome]]s. A multicellular sporophyte generation or phase is present in the life cycle of all land plants and in some algae. 

The sporophyte produces [[spore]]s (hence the name), by [[meiosis]]. These meiospores develop into a [[gametophyte]]. Both the spores and the resulting gametophyte are [[haploid]], meaning they only have one set of [[homologous chromosome]]s. The mature gametophyte produces male or female [[gamete]]s (or both) by [[mitosis]]. [[Sexual reproduction of plants|The fusion of male and female gametes]] produces a [[zygote]] which develops into a new sporophyte. This cycle is known as [[alternation of generations]] or alternation of phases.

In the normal course of events, the zygote and sporophyte will have a full double set of chromosomes again. An exception is when a [[diploid]] and [[haploid]] gamete fuse, resulting in a [[triploid]] sporophyte, which will usually be sterile, as dividing three sets of [[chromosome]]s into two halves causes complications. 

[[Bryophytes]] ([[moss]]es, [[liverwort]]s and [[hornworts]]) have a dominant gametophyte stage on which the adult sporophyte is parasitic. The [[embryo]] of the sporophyte develops from the zygote within the female sex organ or [[archegonium]], and in its early development is therefore nurtured by the gametophyte. Because this embryo-nurturing feature of the life cycle is common to all land plants they are known collectively as the [[Embryophytes]]. 

Most [[algae]] have dominant gametophyte generations, but in some species the gametophytes and sporophytes are morphologically similar ([[isomorphic]]). An independent sporophyte is the dominant form in all [[clubmoss]]es, [[horsetail]]s, [[fern]]s, [[gymnosperm]]s, and [[Flowering plants|angiosperms]] (flowering plants) that have survived to the present day. Early land plants had sporophytes that produced identical spores (isosporous or homosporous) but the ancestors of the gymnosperms evolved complex heterosporous life cycles in which the spores producing male and female gametophytes were of different sizes, the female [[megaspore]]s tending to be larger, and fewer in number, than the male [[microspore]]s. 

During the [[Devonian]] period several plant groups independently evolved [[heterospory]] and subsequently the habit of [[endospory]], in which single megaspores were retained within the sporangia of the parent sporophyte, instead of being freely liberated into the environment as in ancestral exosporous plants. These endosporic megaspores contained within them a miniature multicellular female gametophyte complete with female sex organs or [[archegonia]] containing [[oocytes]] which were fertilised by free-swimming [[sperm]] produced by windborne miniatuarised male gametophytes in the form of pre-[[pollen]]. The resulting [[zygote]] developed into the next sporophyte generation while still retained within the pre-[[ovule]], the single large female meiospore or [[megaspore]] contained in the modified [[sporangium]] or [[nucellus]] of the parent sporophyte. The evolution of heterospory and endospory were among the earliest steps in the evolution of [[seed]]s of the kind produced by [[gymnosperm]]s and [[Flowering plants|angiosperms]] today.   

==References==
*P. Kenrick & P.R. Crane (1997) The origin and early evolution of plants on land. ''Nature'' 389, 33-39.
*T.N. Taylor, H. Kerp and H. Hass (2005) Life history biology of early land plants: Deciphering the gametophyte phase. ''Proceedings of the National Academy of Sciences'' 102, 5892-5897.
*P.R. Bell & A.R. Helmsley (2000) ''Green plants. Their Origin and Diversity.'' Cambridge University Press ISBN 0 521 64673 1

[[Category:Plant morphology]]
[[Category:Plant reproduction]]