One night, after Lewis had spent another wearying day dealing with charges that he’d spent government money on a fur company in which he had an interest, he saw a sealed packet on his desk and immediately recognized Jefferson’s handwriting. Lewis had a smile on his hawk-nosed face as he slid a knife blade under the seal and carefully unwrapped the stiff paper that enclosed a stack of documents. The note inside said:

My Dear Mr. Lewis. Your gardens might benefit from the information contained within. TJ

The next page was entitled “The Cultivation of Artichokes.” The pages contained a detailed treatise, complete with planting tables and a garden diagram.

He spread the contents of the packet on his desktop, his brow wrinkled in puzzlement. Lewis knew of Jefferson’s interest in gardening, but it seemed strange that he would take the trouble to send information on artichoke cultivation halfway across the continent. He must have known that Lewis’s crushing responsibilities left no time for gardening.

Then the light of comprehension dawned on Lewis’s long face and his pulse quickened. He pawed through the shelves of a cabinet where he had stored the reports from the Lewis and Clark expedition and found what he was looking for within minutes.

Sandwiched between two bundles of documents was a sheet of heavy paper, which he extracted and held up to the light. The sheet was perforated with dozens of small rectangular holes, and, with trembling fingers, he placed the matrix over the first written page in the artichoke file and copied the letters that showed through the holes onto a separate sheet of paper.

When Jefferson had conceived the idea for the Pacific expedition, he knew that Lewis would be in a diplomatically sensitive position exploring territory held by France and Spain. Behind Jefferson’s sphinxlike imperturbability was a mind as devious as any to be found in the courts and palaces of Europe. In corresponding with his minister to France, he had often used a cipher, which he described as “a mask when we need it.”

While Lewis was in Philadelphia preparing for his journey with leading scientists from the Philosophical Society, Jefferson sent him a cipher he had worked up for the expedition. He based the encryption method on the Vigenere cipher widely used in Europe. The system involved an alphanumeric table and was unlocked with a key word.

Artichokes.

It had not been necessary to use the cipher during the expedition, which was why Lewis had been baffled at seeing it utilized. Casting questions aside now, he attacked the enciphered message with the enthusiasm he brought to all challenges. As he deciphered each letter from the gibberish, the words began to form before his eyes.