Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/98

for the purposes of nutrition. Starch, composed chemically of carbon and the elements of water (hydrogen and oxygen), its formula being C,,Hj O-, occurs in the form of fine grains, more or less oval or rounded, which vary in diameter from the x^Vcfth to the -ol^tli of an inch. Each grain contains starch in two forms, one, receiving the name gmnulose, is easily soluble, and gives a deep blue colour on the addition of iodine ; the other form, starch cellulose, is less soluble, and gives a yellow or brown colour with iodine ; but the former is most abundant in the grain. The individual grains either lie distinct from each other in the cells, as in the Potato, Wheat, and Fea, or they are aggregated so as to form compound grains, as in West Indian Arrowroot, obtained from Maranta arundinacea, aud Portland Sago, procured from Arum maculatum. Grains of starch pre sent a very characteristic concentrically striated appear ance (fig. 27). This is the result of their mode of for mation. The point round which the stride are arranged is called the hilum. Starch is accumulated iti the internal, and often in the subterranean parts of plants. It occurs abundantly in fleshy roots, and in stems, as well as in seeds and fruits, and is easily separated by washing. The ordinary cultivated grains yield starch in considerable quan tity; so also do the Potato, Arrowroot, and Cassava plants, the Sago-palms, and Banana fruit. That procured from the Arrowroot plant Consists of dull white grains ; while that from the Potato, and from various species of Canna supply ing tous-les-mois, is in the form of large shining par- ticlea Sago aud Tapioca are granulated forms of starch, the former being procured from the cells of various spe cies of Sagus and Metroxylou, the latter from the Cas sava plant The existence of starch in the bark and young wood of trees, such as the Birch and Pine, renders them useful as articles of food in cold countries. Lichenin is a form of starch existing in the cells of Iceland moss and other lichens ; while inulin, which occurs dissolved in the cell-sap, is the starchy matter supplied by the roots of the Dahlia, Dandelion, and Elecampane. By the action of prolonged heat, as well as by the addition of dilute sulphuric acid, or of malt, starch is converted into a soluble gummy substance called dextrin. The same change occurs during germination or the sprouting of the seed. Oily matters occur as drops in the interior of cells, usually associated with starchy substances or with albuminoids. These latter exist as small granules or large rounded masses with definite chemical and optical properties, and are termed aleurone grains. They are frequently associated with a crystalline arrangement of portions of the protoplasm Sugar. pf the cell known as crystalloids. Sugar occurs abundantly in the sap of plants. When pure and in a solid state this substance is crystalline and soluble in water ; but it also occurs in an uncrystallized form. There are two marked varieties of it. Cane-sugar, C 12 H. 2 .O n , procured from the Sugar-cane, Sugar-maple, Beet, Carrot, &c.; and grape- sugar, occurring in numerous fruits, as Grapes, Gooseberries, Currants, Peaches, and Apricots The formula for grape- sugar is C 6 H 12 O ti . During the sprouting of the seed, starch is converted into grape-sugar, and a similar change is in duced by the action of malt, or of any ferment. A sweet substance (not a true sugar), called Mannite, is pro cured from the Manna-ash (Oruus europrca) as well as from various sea-weeds, from species of Eucalyptus, and Gum. from the Dandelion. Gum or mucilage is another substance found in vegetable tissues. When pure it is clear, soluble in water and also in dilute acids, but not soluble in alcohol or ether. It is one of the forms through which vegetable matter passes in being applied to the purposes of plant life. It exists largely in the vegetable juices. From the bark of many trees it is procured in the form of an exudation. Two well-marked kinds of gum are met with, arabin, soluble in cold water, constituting the chief ingredient of gum-arabic, procured from various species of Acacia ; aud cerasin, insoluble in cold water, but readily soluble in boiling water, constituting the gummy secretion obtained from the Cherry and Plum. A substance called bassoriu, or vegetable jelly, is found in Tragacanth, the roots of some Orchids, as well as in Carrageen (Chondrus crispus), and other sea-weeds. It is allied to gum, but differs in swelling up and becoming gelatinous when mixed with water. Another gum-like substance called pectin exists in the juice of the Apple, Pear, and other pulpy fruits. It is changed by the action of alkalies into pectic acid, which is found in many fruits and such succulent roots as Carrot, Turnip, Beet, <kc. Oils, Fats, and Eesins occur in cells of plants, or in special canals or glands as products of assimilation. The oils are either fixed or volatile, the former being divided into drying, fatty, and solid, while the latter are distinguished according as they consist of carbon and hydrogen alone, or of these elements combined with oxygen or with sulphur. Kesinous matter occurs in the form either of fluid balsams, or of the various kinds of solid resin and pitch. In the rind of the Orange and Lemon glands of oil occur (fig. 28). Turpentine canals are met with in the wood of Pines ; and Vittre, or oil-canals, in the fruit of Umbelliferous plants, such as the Coriander. In the fleshy covering of the fruit of the Olive there arc numerous oil-cells. The fruit of the Guinea-palm yields a solid oil, called palm-oil. The dotted appearance of the leaves of the Orange, Myrtle, Eucalyptus, and St John s Wort, depends on the presence of numerous cells or cavities

containing essential oil.

Fig. 27.
[ missing text ]

Fig. 29. Fig. 30. F,-. -].

FIG. 28. Vertical section of part of the rind of the O.angc, showing glands con taining volatile oil, R, it, H, surrounded by cells. Fl ?: 29 - Cells of Kumex f, containing raphldes r. The cells are called Raphi- ,,- ?, "?! are adcular or needle-like crystals forming bundles. MG. 60. Cells of Beet-root, containing conglomerate crystals FIG. 31. Cellular tissue from leaf of Urostigma (Fins) elastiaim. c, a large cell r, cystolith, an agglomeration of crystals situated at the extremity of an inward prolongation of the cell-wall, t ; u, cells filled with chlorophyll granules.

As allied to these secretions, we may notice caoutchouc,

which is found in the milky juice of plants, especially those belonging to the Fig, Spurge, and Dogbane orders. The trees most prolific in this substance are Siphonia elastica, Urceola elastica, and Urostigma elasticum. Gutta- percha is the concrete milky juice of the Tabau plant (Isonandra Gutta). Wax is also found in the tissues of plants, and it frequently occurs as a secretion on the stems, as in the Wax Palm, and on the surface of fruits, as in the bloom or glaucous secretion of the Plum and the Candleberry Myrtle. Crystals of lime salts occur in the

interior of cells, and also in the cell-walls of plants. They