Introduction
_ TO PROFESSOR ASA GRAY THIS VOLUME IS DEDICATED BY THE AUTHOR AS A SMALL TRIBUTE OF RESPECT AND AFFECTION. The subject of the present volume, namely the differently formed flowers normally produced by certain kinds of plants, either on the same stock or on distinct stocks, ought to have been treated by a professed botanist, to which distinction I can lay no claim. As far as the sexual relations of flowers are concerned, Linnaeus long ago divided them into hermaphrodite, monoecious, dioecious, and polygamous species. This fundamental distinction, with the aid of several subdivisions in each of the four classes, will serve my purpose； but the classification is artificial, and the groups often pass into one another. The hermaphrodite class contains two interesting sub-groups, namely, heterostyled and cleistogamic plants； but there are several other less important subdivisions, presently to be given, in which flowers differing in various ways from one anothe

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