A plant that lives for several years, and usually flowers every year. See also: Annual, biennial. Perithecium

A sexually produced fungal body of an Ascomycete that contains one or more asci. Permeability

The ease with which a substance will cross a membrane. For example, polyethylene film is permeable to oxygen and carbon dioxide, but impermeable to water vapour. For this reason, it makes an excellent protective cover for delicate cuttings, etc. Peronospora destructor Downy mildew of onion. Peronospora nicotianae

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