The direct proof that very ancient Romans wrote on the inner bark of trees is preserved in the etymology of the Latin word for book, liber, which originally meant "inner bark" or "bast." Ancient Roman authors and encyclopedists directly recorded this historical fact when discussing the origins of writing materials. [1, 2]

The primary textual evidence comes from Pliny the Elder and Marcus Terentius Varro (as preserved by later grammarians like Servius and Isidore of Seville). [1]

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The Primary Classical Sources

1. Pliny the Elder, Naturalis Historia (Natural History) [1]

In Book 13, Chapter 21, Pliny provides a historical overview of human writing materials. He explicitly lists tree bark (liber) as the surface used by the earliest Romans before the introduction of Egyptian papyrus. [1, 2, 3]

• Latin Text:

  "Antea non fuisse chartarum usum: in palmarum foliis primo scriptitatum, deinde quarundam arborum libris."

• English Translation:

  "Before this, paper was not in use: they first used to write on palm leaves, and then on the inner bark (libris) of certain trees."

  [1, 2]

2. Isidore of Seville, Etymologiae (The Etymologies)

Drawing directly from older, lost Roman works (such as Varro's antiquarian writings), the 7th-century scholar Isidore of Seville explicitly defines why a book is called a liber. [1]

• Latin Text:

  "Liber est interior cortex arboris, qui ligno cohaeret... Hinc et liber dicitur volumen quod nos legimus; nam ante usum chartae vel membranarum volumina ex corticibus arborum conficiebantur, quae libraria dicebantur."

  [1]
• English Translation:

  "Liber is the inner bark of a tree that cleaves to the wood... From this, the volume that we read is called a liber; for before the use of paper [papyrus] or parchment, volumes were made from the barks of trees, which were called libraria."

  [1, 2]

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Archaeological Context

While organic inner bark from the earliest Roman periods has rotted away over the millennia, archaeology has validated the related Roman practice of writing on thin wood sheets. [1]

The famous Vindolanda Tablets discovered along Hadrian's Wall are postcard-sized, leaf-thin slivers of birch and alder wood written on with carbon ink. They offer an intact window into how Romans regularly utilized thin, flexible arboreal fragments for daily correspondence when traditional papyrus was unavailable. [1, 2, 3]

If you are working on a specific text, would you like me to find more parallel quotes from Latin poets (like Vergil or Ovid) who mention carving letters into the bark of living trees?

what would the Varro source be for Isidore, do we know?

Yes, scholars have a very strong indication of what the Varro source would be, though it requires looking into how classical Roman texts were lost and preserved. [1, 2, 3]

Because Marcus Terentius Varro was highly prolific—writing 74 separate works spanning over 600 individual books—the vast majority of his writing has vanished. However, his specific linguistic tracking of words to ancient physical roots is highly preserved. [1, 2]

For Isidore's entry on liber, the primary suspected Varronian origins are split into two categories: extant partial texts and lost monumental encyclopedias. [1, 2]

1. The Surviving Source: De Lingua Latina (On the Latin Language)

This is Varro's multi-volume masterwork exploring the origins and etymologies of Latin terms. Out of the original 25 books, only Books 5 through 10 have survived. [1, 2, 3, 4, 5]

• Why it's likely: In the surviving portions (specifically Books 5 and 6), Varro tackles nouns and how the physical environment, flora, and fauna dictated vocabulary. [1, 2, 3]
• The Connection: While the exact paragraph where Varro explains liber (book) as liber (bark) isn't perfectly spelled out in the surviving fragments, his exact analytical system of tying textual terminology back to arboreal nature appears throughout. Isidore based a massive portion of his etymological frameworks for The Etymologies directly on Varro's framework from De Lingua Latina. [1, 2, 3, 4]

2. The Lost Source: Antiquitates Rerum Humanarum et Divinarum (Antiquities of Human and Divine Things)

If the exact quote did not derive from the remaining books of De Lingua Latina, it almost certainly came from his massive 41-book historical and cultural encyclopedia, the Antiquitates. [1]

• Why it's likely: This specific work detailed the exact everyday lives, materials, religious artifacts, and habits of the earliest prehistoric Romans.
• The Transmission Chain: While the Antiquitates was completely lost to us, it was directly read and heavily cited by subsequent generations of authors, most notably by Pliny the Elder (who openly heavily extracted from Varro for his Natural History) and the grammarian Servius. Isidore compiled his encyclopedia by synthesizing Pliny, Servius, and Suetonius—effectively capturing Varro's data third-hand. [1, 2, 3, 4]

Summary

If you are looking to cite the background of Isidore's statement, you can safely note that it draws upon the linguistic methodology of Varro’s De Lingua Latina, transmitted either directly via surviving antiquarian grammatical chains or filtered through Pliny the Elder’s adaptation of Varro's lost historical encyclopedias. [1, 2, 3, 4]

Would you like help looking at how Pliny the Elder explicitly references Varro in other sections of his books to see how Varro's lost text was reused?