 Four different types of male flowers exist . On the basis of these flower types , the genus has often been split into four genera — a more narrowly defined <unk> , <unk> , <unk> and <unk> . The species sometimes referred to <unk> have coiled anthers , while the other groups have straight ones . The petals of those placed in <unk> are much shorter than the stamens , while those placed in <unk> and a more narrowly defined <unk> have petals that are longer than the stamens . Five species do not fit easily into any of these groups ; this fact has been used as an argument in favour of considering this group a single genus . 
