| Psilotum | ||||||||||||
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 Closeup of Psilotum nudum | ||||||||||||
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Psilotum (whisk fern) is a genus of fern-like vascular plants, one of two genera in the family Psilotaceae, order Psilotales, and class Psilotopsida (the other being Tmesipteris).
Species and distribution
There are two species, Psilotum nudum and Psilotum complanatum, with a hybrid between them known, Psilotum × intermedium W. H. Wagner.
The distribution of Psilotum is tropical and subtropical, in the New World, Asia, and the Pacific. The highest latitudes known are in South Carolina and southern Japan for P. nudum. In the U.S., one species is found from Florida to Texas, the other in Hawaii.
Relation to ferns
The Psilotaceae had traditionally not been thought to be true ferns, but odd "primitive" vascular plants similar to the fossil rhyniophytes and psilophytes. In addition, they lack true leaves and roots (like the Bryophytes), and thus represent the simplest of vascular plants, albeit whose "simple" features are the apparent evolutionary reduction from those of a more complex ancestor.
Recent molecular genetic evidence[1] supports morphological evidence that they may in fact be ferns (Division Pteridophyta) that have lost a number of pteridophytic characteristics.
Gametophyte
The gametophyte generation of Psilotum nudum L. Beauv. (P. triquetrum Swartz) is unusual in having a branching underground structure with vascular tissue in the centre. [2] The nutrition of the gametophyte appears to be myco-heterotrophic, assisted by endophytic fungi.[3]
References
- ^ Qiu, Y-L and Palmer, J (1999) "Phylogeny of early land plants: insights from genes and genomes." Trends in Plant Science 4(1), 26-30
- ^ Holloway, J.E. (1939) The gametophyte, embryo and young rhizome of Psilotum triquetrum Schwarz. Annals of Botany 3, 313-336
- ^ Manton, I. (1942) A note on the cytology of Psilotum with special reference to vascular prothalli from Rangitoto Is. Annals of Botany 6 283-9
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