Found it. Worth a read:
“History records no cassava states, no sago, yam, taro, plantain, breadfruit or sweet potato states,” he writes. What was so special about grains? The answer will make sense to anyone who has ever filled out a Form 1040: grain, unlike other crops, is easy to tax. Some crops (potatoes, sweet potatoes, cassava) are buried and so can be hidden from the tax collector, and, even if discovered, they must be dug up individually and laboriously. Other crops (notably, legumes) ripen at different intervals, or yield harvests throughout a growing season rather than along a fixed trajectory of unripe to ripe—in other words, the taxman can’t come once and get his proper due. Only grains are, in Scott’s words, “visible, divisible, assessable, storable, transportable, and ‘rationable.’ ” Other crops have some of these advantages, but only cereal grains have them all, and so grain became “the main food starch, the unit of taxation in kind, and the basis for a hegemonic agrarian calendar.” The taxman can come, assess the fields, set a level of tax, then come back and make sure he’s got his share of the harvest.