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A nucleus or individual containing only one representative of each chromosome of the chromosome complement. The haploid condition, denoted by the symbol n, is established by mei-otic division of a diploid nucleus. In most plants (bryophytes, ferns, seed-plants, some algae) meiosis establishes a haploid generation, the gametophyte. Sooner or later the haploid gametophyte produces gametes by mitosis. In lower plants the situation is extremely variable. Some, such as Fucus, follow the pattern more characteristic of animals, where meiosis results directly in the formation of gametes. In other cases, such as Spirogyra, the only form of the plant is haploid, and the diploid stage is restricted to a single-celled zygote. In flowering plants the haploid gametophyte generation is reduced to the pollen tube in the male and the embryo sac in the female. However haploid plants can be obtained by culturing pollen grains under suitable conditions. Haploid plants may also be obtained when a zygote formed from an interspecific cross sheds all the chromosomes of one parent as it undergoes development. This phenomenon has been demonstrated when barley (Hordeum vulgare) is fertilized with pollen from the wild barley H. bulbosum. Haploid plants have great potential in plant breeding as it is possible, by doubling the chromosomes of a haploid plant, to obtain a completely homozygous plant. This may be impossible by other means, especially with self-sterile plants.
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