R̃ (lower case: r̃), is a Latin R with a diacritical tilde. In the International Phonetic Alphabet it represents a nasalized version of a trill consonant, or in broader transcription also a nasalized flap or tap.
It is not to be confused with Ȓ (R with inverted breve), R̄ (R with macron) or Ř (R with háček).
Well, that’s interesting to know that Psilotum nudum are known as whisk ferns. Psilotum nudum is the commoner species of the two. While the P. flaccidum is a rare species and is found in the tropical islands. Both the species are usually epiphytic in habit and grow upon tree ferns. These species may also be terrestrial and grow in humus or in the crevices of the rocks.
View the detailed Guide of Psilotum nudum: Detailed Study Of Psilotum Nudum (Whisk Fern), Classification, Anatomy, Reproduction