A cryptogam (scientific name Cryptogamae) is a plant (in the wide sense of the word) that reproduces by spores, without flowers or seeds. "Cryptogamae" (Greek κρυπτός kryptos, "hidden" + γαμέω, gameein, "to marry") means hidden reproduction, referring to the fact that no seed is produced, thus cryptogams represent the non-seed bearing plants. Other names, such as "thallophytes", "lower plants", and "spore plants" are also occasionally used. As a group, Cryptogamae are the opposite of the Phanerogamae (Greek φανερός, phaneros = "visible") or Spermatophyta (Greek σπέρμα, sperma = "seed" and φυτόν, phyton = "plant"), the seed plants. The best known groups of cryptogams are algae, lichens, mosses and ferns, but it also includes non-photosynthetic organisms traditionally classified as plants, such as fungi, slime molds, and bacteria.
At one time, the cryptogams were formally recognised as a group within the plant kingdom. In his system for classification of all known plants and animals, Carl Linnaeus (1707–1778) divided the plant kingdom into 24 classes, 
During World War II, the Government Code and Cypher School recruited Geoffrey Tandy, a marine biologist expert in cryptogams, to Station X, Bletchley Park when someone confused these with cryptograms.
- Plant – to see how cryptogams are distributed across modern classification systems
- "Cryptogams". Royal Botanic Garden, Edinburgh. Archived from the original on 2007-11-18. Retrieved 2007-07-02.
- Gilbert M. Smith (1938). Cryptogamic Botany, Vol. 1. McGraw-Hill. Retrieved September 2014. Check date values in:
- P. S. Dixon (1973). Biology of the Rhodophyta. Oliver and Boyd, Edinburgh. ISBN 0-05-002485-X.
- Smithies, Sandy (19 January 1999). "Television Tuesday Watching brief". The Guardian. Retrieved 23 July 2015.
- Davies, Mike. Cracking the code at last of Station X Birmingham Post 20 January 1999
- Hanks, Robert. Television Review The Independent 20 January 1999
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