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The polypetalous genera Pyrola, Ledum, and Monotropa among the sympetalous heaths, are a remarkable instance of this. The whole evolution of the monocotyledons from the lowest orders of dicotyledons implies the seeming loss of cambial growth and many other qualities. Degressive evolution is not so common by far, and is not so easy to recognize, but no doubt it occurs very frequently.
Such cases of atavism may be observed either as specific marks, or in the way of anomalies. Ledum, Monotropa and Pyrola, or the Labrador tea, the Indian pipe and wintergreen are instances of reversionary gamopetalism with free petals. Sometimes it exhibits five free petals, while at other times only one or two are entirely free, the remaining four being incompletely loosened.
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