What then unites these diverse plants into the family called the Orchidaceae?

All orchids have protocorms, and these are not found in any other family. A protocorm is the structure formed after the germination of the seed and before the development of the seedling plant. The protocorm has no radicle and instead has mycotrophic tissue (and hence differs from other flowering plant seedlings). Other important characters shared by most orchids include: * The fusion of the male and female organs into a single structure, called the column. * A large number of small seeds per ovary. * Stamens are found on the abaxial side of the flower (the side facing downwards/away from the stem). * The lip/labellum (a modified petal) occurs opposite the fertile stamen(s). * Flowers are often resupinate (have twisted through 180° during development). * Pollen is usually bound together to form large masses (called pollinia).  more