The virus that made the price 

The tulips that commanded the highest prices in the 1630s were not the clean single-colour blooms. They were the ones the Dutch called broken: a base colour split by flames, feathers and streaks of white or contrasting pigment, no two flowers quite alike. Semper Augustus, the most famous of all, was a white bloom shot through with red.

Nobody could produce them on demand. A bed of ordinary red tulips might throw a broken bloom one season for no visible reason, and the break would then pass into that bulb's offsets. Growers watched their fields the way prospectors watch a river. The literature of the period is full of speculation about soil, manure, planting depth and the age of the bulb, none of which was the answer.

The answer, established in the twentieth century, is a virus. Tulip breaking virus is a potyvirus carried between plants by aphids. It disrupts the layer of pigment in the petal, letting the underlying white or yellow show through in irregular bands. That is the flame. It is not a breeding achievement, it is an infection, and it is why no grower of the 1630s could work out the mechanism.

There is a second consequence that matters more for the economics. Infected bulbs are weakened. They tend to be smaller, they flower less vigorously, and crucially they produce fewer offsets than healthy stock. So the varieties in highest demand were precisely the ones least able to expand their own supply. Fashion pointed at the flowers biology had handicapped.

This is a good moment to be careful, because the fact is often stretched. The virus does not mean the whole episode was irrational. A broken tulip was genuinely rare, genuinely beautiful, and genuinely hard to reproduce, so paying a lot for a named specimen was a defensible thing for a wealthy collector to do. Nor was every high price in the period attached to a broken bulb. By the winter of 1636 and 1637 much of the trading volume had moved into cheaper, plainer bulbs sold by weight, which is a different market with different buyers.

What the virus does give you is the clean version of a pattern that repeats. The asset at the centre of a boom is frequently not understood by the people trading it. The market had a working theory of value, based on visible beauty and observable rarity, and no theory at all of causation. It could price a broken tulip without knowing what a broken tulip was.

Which raises the question every retelling reaches for: how high did those prices actually go, and what would they have bought instead?