The milky latex in rubber gloves evolved as a sticky plant weapon
Snap a milkweed leaf and white fluid wells up. That latex is not sap. It is a defensive cocktail packed with toxins that clots in air, gumming up insect mouthparts, and in one milkweed it traps and kills around 30 per cent of newly hatched monarch caterpillars. Humans turned the same fluid into rubber.
Latex, from the Latin for liquid, is technically an emulsion: tiny polymer particles suspended in water. In nature it oozes from about 10 per cent of flowering plants after injury, in roughly 20,000 species from more than 40 families, and also from some mushrooms. Most latex is white, but some runs yellow, orange or scarlet, and the mushroom Lactarius indigo bleeds vivid blue. Because such distant lineages make it, biologists see it as a trait that evolved independently many times.
Plants build the plumbing in two ways. In poppies and rubber trees, rows of cells dissolve their shared walls to form continuous tubes. In milkweeds and spurges, a latex cell present in the embryo keeps growing and branching until a single cell can thread through the whole plant.
The case for defence is strong. The idea was proposed in 1887 by Joseph F. James, who judged milkweed's distasteful fluid better protection than any thorns or hairs. Slugs will eat leaves drained of latex but refuse intact ones, and many insects carefully cut the latex-carrying veins before feeding. Latex holds defensive chemicals at 50 to 1000 times the concentration found in other tissues, and plants push it toward a wound; in 1935 the botanist Catherine Bangham saw a pierced Cryptostegia stalk shoot a jet more than a metre long. Older theories about storing nutrients or balancing water lack evidence.
Human uses are many. Chicle latex went into chewing gum, dried opium poppy latex yields morphine and codeine, and synthetic latex binds paints and glues that dry without toxic solvents. Natural rubber is the prize: over 12,000 plants make rubber-bearing latex, though few are commercially useful. Latex allergy is a real problem, and many allergic people also react to fruits such as banana, avocado and kiwi, a cross-reaction first described in 1994. The guayule shrub's latex carries only 2 per cent of the protein of standard rubber-tree latex.
Source: Latex
