Historical concepts of the velamen and exodermis are reviewed. 2. Twenty species and hybrids, representing thirteen genera of orchids, were investigated to determine the origins of the velamen and exodermis. 3. Fourteen orchid root tips belong to the category of histogens known as type I; the other six belong to Haberlandt's type VI. 4. The velamen is derived from the protoderm. It is a multiple epidermis. 5. The exodermis is the specialized outermost layer of the cortex. It corresponds, in its limiting position and similar ontogeny, to the endodermis.
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Charles J. Engard (1944) studied this question.
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