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Psilotum (whisk ferns; family Psilotaceae) A genus of very primitive vascular plants without true roots, and with the spore capsules borne in fused triads on the upper parts of the forking stems, in the axils of tiny scales. There are no real leaves, but the ridged stems are green, and act as photosynthetic organs. There are 2 species, one (P. nudum) occurring widely in the tropics and subtropics, including Australasia and the Pacific islands, extending into southern Spain and into the southern part of the USA; the other, P. complanatum, is found in Mexico, Jamaica, and the Pacific.