Glossary of "Plant Biology (1st ed.)" by Graham et al. (2003)
- The outermost, primary tissues of plants; located at the surfaces of leaves and young roots and stems.
Glossary of "Life (9th ed.)" by Sadava et al. (2011)
- In plants and animals, the outermost cell layers. (Only one cell layer thick in plants.)
Glossary of Plant Systematics (1st ed.) by Simpson (2006)
- The outermost cell layer of all land plant organs, functioning to provide mechanical protection of inner tissue and to inhibit water loss.
Glossary of "A Botanist's Vocabulary" by Pell & Angell (2016)
- Graham, L. E., Graham, J. M. & Wilcox, L. W. 2003. Plant Biology. -xxxv + 497 pp. Pearson Education, Inc., Upper Saddle River.
- Sadava, D., Hillis, D. M., Heller, H. C. & Berenbaum, M. R. 2011. Life: The Science of Biology, 9th ed. -xliv + 1266 pp. Sinauer Associates, Inc., Sunderland.
- Simpson, M. G. 2006. Plant Systematics. -xi + 590 pp. Elsevier Academic Press, Oxford.
- Pell, S. K. & Angell, B. 2016. A Botanist's Vocabulary: 1300 Terms Explained and Illustrated. 226 pp. Timber Press, Portland.