_Gum_ is excreted from various plants as a thick fluid, which dries up into transparent masses. Its composition is identical with that of starch. It dissolves readily in cold water, and is converted into sugar by long continued boiling with acids. Its properties are best marked in gum arabic, which is obtained from various species of acacia; that from other plants differs to some extent, although its chemical composition is the same. _Dextrine._--When starch is exposed to a heat of about 400 deg., or when treated with sulphuric acid, or with a substance extracted from malt called _diastase_, it is converted into dextrine. It may also be obtained from cellulose by a similar treatment. The dextrine so obtained has the same composition as the starch from which it is produced, but its properties more nearly resemble those of gum. It plays a very important part in the process of germination, and may be converted into sugar on the one hand, and apparently also into starch on the other. _Sugar._--Under this name are included four or five distinct substances, of which the most important are, cane sugar, grape sugar, and the uncrystallisable sugar found in many plants. _Cane Sugar._--This variety of sugar, as its name implies, is found most abundantly in the sugar cane, but it occurs also in the maple, beet-root, and various species of palms, from all of which it is extracted on the large scale. It is extremely soluble in water, and can be obtained in large transparent prismatic crystals, as in common sugar-candy. It swells up, and is converted into a brown substance called caramel, when heated, and by contact with fermenting substances, yields alcohol and carbonic acid. It contains-- Carbon 42.22 Hydrogen 6.60 Oxygen 51.18 ------ 100.00 and its chemical formula is C_{12}H_{11}O_{11}. _Grape Sugar_ is met with in the grape, and most other fruits, as well as in honey. It is produced artificially when starch is boiled for a long time with sulphuric acid, or treated with a large quantity of diastase. It is less soluble in water than cane sugar, and crystallises in small round grains. Its composition, when dried at 284 deg., is-- Carbon 40.00 Hydrogen 6.66 Oxygen 53.34 ------ 100.00 and its formula is C_{12}H_{12}O_{12}; but when crystallised it contains two equivalents of water, and is then represented by the formula C_{12}H_{12}O_{12} + 2H_{2}O. The uncrystallisable sugar of plants is closely allied to grape sugar, and, so far as at present known, has the same composition, although, from the difficulty of obtaining it quite free from crystallised sugar, this is still uncertain.