Telling Norway maple from sugar maple in the summer can be tricky as their leaves look nearly identical, at least to the untrained eye (take the sugar vs Norway maple leaf ID quiz to see how good you are at this). While there are plenty of non-leaf features you can use, one of the more reliable diagnostic characteristics of Norway maple is the milky, opaque sap that oozes from a broken leaf (or twig or root).
The first photo below I took on June 1, 2018, well after leaf out, and the twig below that I cut and photographed on November 22, 2018, both of which show white sap, though the leaf is clearly whiter and more opaque. As I wrote in the last newsletter (A year in the life of sap), sap has to play the duel role of energy storage and chemical defense against predators. But this is primarily only important during the growing season. The white is related to predator defense (more on this below), so by early winter, the sap in twigs of Norway maple become increasingly clear and by early spring, the sap is crystal clear and totally delicious.
Technically, it’s not the sap itself that is white. The sap of these plants is still clear throughout the growing season. Instead it’s a specialized latex produced by these plants during the growing season that is infused into the sap, giving it a thick milky appearance. Latex is primarily a defensive compound made up of a cocktail of secondary metabolites (these organic compounds perform a diverse array of functions not related to normal growth, development and reproduction). Latex is produced in specialized cells throughout the plant, and these cells are under great pressure and so that when a plant stem breaks, the the latex is released and exposed at the damaged site, which presumably is where predators are attacking or will soon attack.
About 10% of all flowering plants exude latex (source), though the number is lower, about 6%, in temperate climates where herbivore-pressure is lower (source). You’re likely familiar with many of these latex-producing plants: milkweed, dogbane, dandelion, bloodroot, celandine, periwinkle, and wild lettuce, among many others. The number of latex-producing trees here in New England is a bit smaller (<5% have milky sap): Norway maple, staghorn sumac (the sap is white in growing season, but becomes a thick orange color by winter), and mulberry produce latex.
Two side notes: Dogwoods have latex in their leaves, which becomes solid and stringy when exposed to the air (see image at bottom). While hackberry is in the family Cannabaceae, along with marijuana, which produces latex, hackberry does not.