Part 67. Plant Surface Fibers

Part 67. Plant surface fibers occur upon stems, leaves, fruit, and seeds. The only one of much economic importance is cotton. This forms the woolly covering of the seeds of several species, principally the upland cotton (Figs. 214, 215) and the Sea Island cotton (Fig. 216). The former is the chief fiber plant of the world. To-day it is cultivated throughout the tropics and very generally in subtropical regions. The southern United States produce more than all the rest of the world. Two thousand six hundred years ago it was raised in India whence its culture slowly spread. It was introduced into this country in 1774. Long before the coming of Columbus, however, Sea Island cotton was raised by the natives of tropical America.

Fig. 214.-Upland Cotton (Gossypium herbaceum, Mallow Family, Malvaceae). Plant in flower and fruit. (Baillon.)-An annual in cultivation, growing 1-2 m. tall; leaves downy; flowers yellow; fruit dry; seed brown. Native home, probably India.

Fig. 215.-Upland Cotton. A, flower-bud. B, flower. C, unopened pod. D, open pod, displaying the masses of hairy seeds. E, pistil. F, stamen. G, ovary, cut across. H, seed showing mass of hairs (cotton). J, same, cut vertically, showing the much-folded embryo. (Baillon.)

Fig. 216.-Sea Island Cotton (Gossypium barbadense, Mallow Family, Malvacece). Flowering top, 1/4. (Schumann.)-Similar to upland cotton but with seed black. Native home, West Indies.

Upland cotton yields much the larger amount of fiber which although strong is only about 1-2 cm. long. The Sea Island cotton has a finer fiber, about 2.5-4 cm. long, and is therefore the more valuable; but the yield of the plant is comparatively small and the cultivation is mostly confined to islands or regions near the coast.

The fibers found on the seeds of each consist of simple, flattened and twisted hairs developed as outgrowths from the "hull" or seed coat. In the wild state these hairs catch the wind like thistle-down and so are of service to the plant as means of spreading abroad its seeds. Cotton raisers, however, select varieties which hold their seeds firmly in the pod till they can be picked out by hand. Through selection also the best varieties have lost the yellowish or buff color of their seed hairs, and have become nearly or quite white. At the same time there has been developed in cultivation the remarkable twist before mentioned. This twist is of the highest importance. It favors the interlocking of the fibers in spinning and thus makes possible yarns which combine in a wonderful way extreme fineness, softness, and strength. No other fiber has this peculiar twist. Of all others the wool of sheep, from its curliness, most nearly resembles cotton, which indeed well deserves to be called "vegetable wool."

The separation of the fiber from the seed after picking is accomplished by a machine called a gin which either pulls the seed from the fiber by means of rollers, or tears away the fiber by the action of notched wheels revolving rapidly between the bars of a grating too narrowly set for the seeds to pass through. The ginned fiber is ready for spinning after various machines have removed impurities, and combed the fibers approximately parallel. After spinning, the yarn is bleached, or dyed if necessary, and may be then twisted into thread or other cordage, or may be woven or otherwise made into a fabric. The cleaned fiber rolled into sheets is cotton batting, widely used for filling. In their crude state, cotton fibers are covered with an oily varnish which repels water. When this layer is removed and the fibers thoroughly cleansed there is obtained a white, fleecy mass which is highly absorptive. This is extensively employed in medicine and surgery under the name "absorbent cotton." Like the best paper it is nearly pure cellulose. Many of the finer sorts of paper are made from cotton rags, waste from spinning mills, and fibers too short to spin.

Absorbent cotton treated with nitric and sulphuric acids becomes converted into nitrocellulose or guncotton. An intimate mixture of this with laurel camphor forms celluloid. Collodion is a form of nitrocellulose dissolved in ether and alcohol. Artificial silk is made by forcing collodion through exceedingly fine openings into running water, where the collodion at once hardens into a silky fiber, which after thorough washing becomes well adapted to the same uses as natural silk. The carbon filaments of incandescent electric lamps are charred cotton threads or sometimes carbonized strips of paper pulp. Cotton is the fiber chiefly used also for candle and lamp wicks.