text:  <unk> exists as to whether <unk> should be considered a single genus , or a group of related genera . In their 1996 Field Guide to the <unk> of the Americas , Andrew Henderson , Gloria <unk> and <unk> <unk> combined all the species in the <unk> <unk> ( as it was then defined ) into a single genus , <unk> . In his 1999 <unk> Treatment of Palm <unk> <unk> , American botanist Sidney F. <unk> divided the group into five genera — a more narrowly defined <unk> , <unk> , <unk> , <unk> and <unk> . <unk> <unk> and John <unk> recognised a single genus in their 2005 World <unk> of <unk> , and Jean @-@ Christophe <unk> continued this usage in his 2008 review of the genus . 
  