Most people are familiar with the regular arrangement of the segments of the flowers of Amaryllids and Lilies, with their prominent pistils and anthers. The first stage in the advance of the Orchid family is shown in the Apostasieæ, comprising Apostasia, Neuwiedia and Adactylus, in which the perianth segments are more or less regular and the anthers in some degree prominent, Neuwiedia, with its free stamens and prominent style, appearing at first sight nearer to some of the Amaryllids than to the Orchideæ commonly seen in gardens.
The Cypripedieæ, although so widely separated from other sections as to suggest that in the operations of nature a vast number of connecting types must have become extinct, is the next step, the labellum being formed into a pouch with infolded side lobes. The column has a prominent staminode with two fertile anthers below it, one on each side of the column and behind the stigmatic plate. The upper sepal is frequently the showiest feature in the flower; the lower sepals are joined and arranged behind the lip, whilst the petals extend on each side and vary much in form.
In gardens, the whole of the genus is known as Cypripedium, although the South American species (Selenipedium), having a three-celled ovary, differ widely from the one-celled East Indian and Malayan species, and other sections have such marked and consistent botanical differences as to warrant the botanist in separating them under different sub-generic names. The third section of Orchidæ, the largest family of the Monocotyledons, forms the chief class of Orchids as they are known in gardens. In this class the stamens and style unite into a column, and at the top of the column the pollen masses are situated; these are covered by the anther-cap, and in a cavity is the stigma with its viscid surface to receive the pollen grains.
So diverse and intricate are the forms of the flowers, and especially labellums, that there is little doubt that insect aid is necessary in their natural habitats to bring about pollination. It has been proved by the operations carried out in cross-fertilisation in gardens that no class of plants can be so readily crossed under artificial conditions. It is not necessary here to go further into structural details, as the peculiarities of each section will be remarked on under their different headings. But it may be said that in what are called abnormal flowers, which have perfect stamens and style, can be seen instances suggesting the evolutionary process; these would be more common but for the number of connecting links which have dropped out in the great struggle for existence.