These are combustible compounds, but are neither starchy, saccharine, nor oily. Gum is found in apple and plum trees. Gum arabic is from a species of acacia, and is soluble. Mucilage is found in onions, quinces, and flaxseed. It forms a jelly with water, but does not dissolve like gum arabic. Pectose is found in many roots, like the turnip; also in the pear and peach. When boiled with water it changes to a vegetable jelly, called pectine. Currants, pears, peaches, plums, contain pectine, and this is what gives firmness to the preserves made from them. Cellulose is the woody fibre of stalks of grain, the membrane which envelops the grains, husks, and skin of seeds, rinds, cores, and stones of fruits. It is the main and almost the sole constituent of linen, and of the paper which is made from linen.