Cellulose, the other major cell-wall component, is very resistant to change, and this is one reason that it’s the most abundant plant product on earth. Like starch, cellulose consists of a chain of glucose sugar molecules. But a difference in the way they’re linked to each other allows neighboring chains to bond tightly together into fibers that are invulnerable to human digestive enzymes and all but extreme heat or chemical treatment. Cellulose becomes most visible to us in the winter as hay, a stubble field, or the fine skeletons of weeds. This remarkable stability makes cellulose valuable to long-lived trees and to the human species as well. Wood is one-third cellulose, and cotton and linen fibers are almost pure cellulose. However, cellulose is a problem for the cook: it simply can’t be softened by normal kitchen techniques. Sometimes, as in the gritty “stone cells” of pears, quince, and guava, this is a relatively minor distraction. But when it’s concentrated to provide structural support in stems and stalks—in celery and cardoons, for example—cellulose makes vegetables permanently stringy, and the only remedy is to pull the fibers from the tissue.