As part of his Comparative Investigations of the Organization of the Trunk of the Native Forest Trees (Theodor Hartig 1837, Vergleichende Untersuchungen tiber die Organisation des Stammes der einheimischen Waldbaume. lahresberichte tiber die Fortschritte der Forstwissenschaften und forstlichen Naturkunde 1: 125-168) Hartig gives an anatomical description of the "composition and nature" of the then "completely uninvestigated elementary organs" of what he called the "sap skin" (Safthaut) of trees, a tissue for which Nageli later (1858) coined the term phloem. Within the "Safthaut" Hartig describes three cell types in detail, (1) "Siebfasern", (2) "Siebrohren", and (3) "keulenfOrmige Saftrohren" (club-shaped sap-tubes). While the description of the latter refers to laticifers in Euphorbia and resin ducts in Acer and Robinia. "Siebfasern" and "Siebrohren" comprise the sieve elements. A literal translation of the more significant parts of the description of these cell types demonstrates that his "Siebrohren" entirely correspond to what has later been defined as "sieve tubes" but that his "Siebfasern" are less well- defined and in addition to what we call "sieve cells" also include small sieve tubes as well as spindle-shaped cells of cambium, phloem parenchyma and sclerenchyma. Both in his "Siebfasern" and "Siebrohren" Hartig describes sieve areas (his expression is "lense-shaped cavities") and sieve pores (Siebporen).
