_Mucilage_ is the name applied to the substance existing in linseed, and in many other seeds, and which communicates to them the property of swelling up and becoming gelatinous when treated with water. It is found in a state of considerable purity in gum tragacanth and some other gums. Its composition is not known with absolute certainty, but it is either C_{24}H_{19}O_{19}, or C_{12}H_{10}O_{10}; and in the latter case it must be identical with starch and gum. It will be observed that all the substances belonging to this class are very closely related in chemical composition, some of them, as starch and gum, though easily distinguished by their properties, being identical in constitution, while others only differ in the quantity of water, or of its elements which they contain. In fact, they may all be considered as compounds of carbon and water, and their relations are, perhaps, more distinctly seen when their formulae are written so as to show this, as is done in the following table, in the second column of which those containing twelve equivalents of carbon are doubled, so as to make them comparable with cellulose:-- Water. Grape sugar, C_{12}H_{12}O_{12} C_{24}H_{24}O_{24} C_{24} + 24 Cane sugar, C_{12}H_{11}O_{11} C_{24}H_{22}O_{22} C_{24} + 22 Cellulose, C_{24}H_{21}O_{21} C_{24}H_{21}O_{21} C_{24} + 21 Inuline, C_{24}H_{21}O_{21} C_{24}H_{21}O_{21} C_{24} + 21 Starch, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20 Dextrine, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20 Gum, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20 Mucilage, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20 The relation between these substances being so close, it is not difficult to understand how one may be converted into another by the addition or subtraction of water. Thus, cellulose has only to absorb an equivalent of water to become grape sugar, or to lose an equivalent in order to be converted into starch, and we shall afterwards see that such changes do actually occur in the plant during the process of germination. _Pectine and Pectic Acid._--These substances are met with in many fruits and roots, as, for instance, in the apple, the carrot, and the turnip. They differ from the starch group in containing more oxygen than is required to form water along with their hydrogen; but their exact composition is still uncertain, and they undergo numerous changes during the ripening of the fruit.