The Expanded Adaxial Epidermis of Equisetum Rhizome Sheath Teeth

Abstract

Plants of Equisetum have an extensive rhizome system that produces nodal buds that grow into either aerial stems or branches of the rhizome. Certain species also have tuber buds. Both the aerial stems and the rhizomes of Equisetum have nodal sheaths bearing a crown of teeth. The sheath is interpreted as a whorl of fused leaves, with the teeth representing the free tips of the individual leaves. The buds consist of a series of superposed sheaths that overarch the shoot apex. Subsequent intercalary meristematic activity at the base of each nodal sheath separates the nodal sheaths and produces the internodes. The sheath teeth of the subterranean buds have a distinctive adaxial epidermis, a feature rarely noted in the literature. Francini (1942) compared rhizome and aerial buds of E. ramosissimum, and described trichomes developing from the inner epidermis of the rhizome sheath teeth and growing together to help the teeth form a cap protecting the apex. She also described mucilage production from the abaxial epidermis of the outer sheaths and pointed out how only one internode at a time elongates in the rhizome. Sachs (1882, p. 401) included a figure of a longitudinal section through an underground bud of Equisetum arvense showing an elaboration of the adaxial surface of the sheath teeth, but he did not describe this feature. Sadebeck (1902, p. 532) copied Sachs' figure, but did not mention the structure of the adaxial surface of the teeth. The purpose of this study is to describe more completely this little noted anatomical feature of Equisetum, and to speculate on its function.