Two genera of rein-orchids occur in the Pacific Northwest (Piperia Rydb. and Platanthera L.C. Rich.), both of which have been subsumed by some authors under the genus Habenaria Willd. Plants of the genus Piperia can be distinguished from these in Platanthera by the following characters:

1. terrestrial rather than semi-aquatic habitat
2. stem leaves reduced to bracts
3. basal leaves usually withering before or during anthesis
4. stem arising from an ovoid tuber rather than fusiform roots
5. lip with median ridge rathe
6. anther cells opening laterally rather than apically
7. flowers protandrous by movement of the lip

Spur 2-6 mm, more or less shorter than lip
• Flowers white, blade of lip becoming curved downward and back toward spur
  

  ............................ P. candida Morgan & Ackerman

• Flowers green to translucent, blade of lip straight or curving slightly upward at tip
  

  ......................... P. unalascensis (Spreng.) Rydb.

Spur 7-15 mm, longer than lip
• Perianth green or partially translucent; viscidia oval, not much longer than wide; plant generally tall, slender
  

  ....................................... P. elongata Rydb.

  
  
  Perianth parts white with green or yellow-green midvein, viscidia oblong, about twice as long as wide; plants robust to delicate
• Plants relatively delicate (stem 1-3 mm diam.); spur straight, held horizontally; lip projecting forward; floral fragrance clovelike, nocturnal
  

  ............................... P. transversa Suksdorf

• Plants relatively robust (stem usually more than 3 mm diam.); spur usually more or less curved, often held concealed against stem; lip curving downward in age; floral fragrance often strong, but not clovelike
  

  ............................ P. elegans (Lindl.) Rydb. (syn.: P. maritima [Greene] Rydb.)