The dimorphic hypodermis is composed of two kinds of cells differing in size and staining properties, often alternating in a regular pattern. The roots of 98 of 358 genera examined for the presence of a dimorphic hypodermis had one and 97 additional genera with a dimorphic hypodermis were compiled from the literature. The hypodermal type of the root was found to be a fairly constant characteristic at the family level. Forty-one angiosperm families had members with a distinct dimorphic hypodermis. A dimorphic hypodermis was found both in families with many specialized characteristics such as the Orchidaceae and in families which had many unspecialized characteristics such as the Magnoliaceae.