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The sequence of orders in the polypetalous subdivision of Dicotyledons undoubtedly represents a progression from simpler to more elaborate forms, but a great drawback to the value of the system is the inclusion among the Monochlamydeae of a number of orders which are closely allied with orders of Polypetalae though differing in absence of a corolla.
In the polypetalous forms progression from hypogyny to epigyny is generally recognized, and where dorsiventrality with insect-pollination has been established, a dominant group has been developed as in the Leguminosae. The starting-point of the class, however, and the position within it of apetalous families with frequently unisexual flowers, have provoked much discussion.
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.
If the character became fixed, so as to lose its present state of variability, such a group of supposititious gamopetalous plants might be quite analogous to the corresponding real gamopetalous families. Hence there can be no objection to the view, that the heaths have arisen in an analogous manner from their polypetalous ancestors.
A child who loves flowers goes to school; he is given one of his favorites and told to pull it to pieces, look at its different parts, and label them with such words as petals, sepals, pistil, stamens; to these are presently added calyx, corolla, monopetalous, polypetalous, innate, adnate, indehiscent, etc., until the child's mind resembles a lumber room of senseless rubbish, in which the flower is buried and lost.
Perching himself on the mossy stump, Ben obediently floundered through the following analysis, with constant help in the spelling, and much private wonder what would come of it: "Phaenogamous. Exogenous. Angiosperm. Polypetalous. Stamens, more than ten. Stamens on the receptacle. Pistils, more than one and separate. Leaves without stipules. Crowfoot family. Genus ranunculus.
Some represent natural groups and had in several cases been already recognized by Ray and others, but the majority are, in the light of modern knowledge, very mixed. Well-defined polypetalous and gamopetalous genera sometimes occur in the same order, and even Monocotyledons and Dicotyledons are classed together where they have some striking physiological character in common.
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