This section is from the book "Who's Who Among The Wild Flowers", by W. I. Beecroft. Also available from Amazon: A Field Guide to Edible Wild Plants.
Large Yellow Lady's Slipper: Yellow Moccasin Flower: Whippoorwill's Shoe.
Cypripedium parviflorum Var. pubescens. Orchis Family. May - July.
Found in moist or boggy woods. Nova Scotia to Alabama, west to Minnesota, E. Kansas and Nebraska.
Solitary, borne at the top of leafy stem 1 - 2 feet high; pouch golden yellow; other divisions of flower lined and marked with madder-purple.
Parallel veined, clasping stem.

Yellow Lady's Slipper.
C. parviflorum. Similar but smaller than the preceding, with similar range. Intergrading forms frequent.
 
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