root bearing at the apex a stem bud. As in the case of the stem,
growth in length occurs only for a short distance behind the apex,
Fig. 6.—Base of
plant of Orchis,
showing tubercules
or tuberous
roots.
but in long-lived roots increase in diameter
occurs continually in a similar manner to
growth in thickness in the stem.
Roots are usually underground and colourless, but in some cases where they arise from the stem they pass for some distance through the air before reaching the soil. Such roots are called aerial. They are well seen in the screw-pine (Pandanus), the Banyan (Ficus indica, fig. 7), and many other species of Ficus, where they assist in supporting the stem and branches. In the mangrove they often form the entire support of the stem, which has decayed at its lower part. In tree-ferns they form a dense coating around, and completely concealing, the stem; such is also the case in some Dracaenas and palms. In Epiphytes, or plants growing in the air, attached to the trunks of trees, such as orchids of warm climates, the aerial roots produced do not reach the soil; they continue always aerial and greenish, and they possess stomata. Delicate hairs are often seen on these epiphytal roots, as well as a peculiar spongy investment formed by the cells of the epidermis which have lost their succulent contents and are now filled with air. This layer is called the velamen, and serves to condense the moisture contained in the air, on which the plant is dependent for its water-supply. The aerial 'roots of the ivy are not the nutritive roots of the plant, but are only intended for mechanical support. The climbing roots of many orchids, aroids and epiphytic ferns branch and form places of lodgment for humus into which absorbent branches of the climbing roots penetrate. Some leafless epiphytic orchids, such as species of Angraecum, depend entirely upon their aerial roots for nourishment; the roots, which are green, perform the functions both of leaves and roots. A respiratory or aerating function is performed by roots of certain mangroves growing in swampy soil or water and sending vertical roots up into the air which are provided with aerating passages by which the root system below can communicate with true outside air.
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Fig. 7.—Ficus indica, the Banyan tree, sending out numerous aerial roots, which reach the soil, and prop the branches. |
Parasitic plants, as the mistletoe (Viscum), broom-rape (Orobanche) and Rafflesia, send root-like processes into the substance of the plants whence they derive nourishment. In the dodder (Cuscuta), the tissue around the root swells into a kind of sucker (haustorium), which is applied flat upon the other plant, and ultimately becomes concave, so as to attach the plant by a vacuum. From the bottom of the sucker the root protrudes, and penetrates the tissue of the host plant. Leaf-buds are sometimes formed on roots, as in plum, cherry and other fruit trees; the common elm affords an excellent example, the young shoots which grow up in the neighbourhood of a tree arising from the roots beneath the soil. In some plants no roots are formed at all; thus in the orchid Corallorhiza, known as coral-root, a stem-structure, the shortly branched underground rhizome, performs all the functions of a true root which is absent. In aquatic plants the root acts merely as a holdfast or is altogether absent as in Salvinia, Utricularia and others.
ROPE AND ROPE-MAKING. All varieties of cordage having a circumference of an inch or more are known by the general name of “rope.” Twisted cordages of smaller dimensions are called cords, twines and lines, and when the sectional area is still smaller, the article is known as thread or doubled yarn. All these varieties of cordage are composed of a number of separate yarns, each of which is made from some kind of textile fibre by preparing and spinning machinery. The number of separate yarns which ultimately form the rope or cord depends upon the fineness of the yarn, and also upon the circumference of the finished article. From thread and fine twine upwards the whole art of manufacture is that of twisting together fibres and yarns; but the comparative heaviness and coarseness of the materials operated on in rope-making render necessary the adoption of heavy machinery and modified processes which clearly define this manufacture as a distinct calling. The modern trade of rope-making is again divided into two distinct branches dealing with vegetable fibres and metallic wire.
Many different vegetable fibres are used for rope-making, but for the combined qualities of strength, flexibility and durability, none can compete with the common hemp, which is consequently the staple of the rope-maker. Cotton ropes are, however, much more flexible, and in addition are strong and durable; they are, therefore, much preferred for power transmission in textile and other works. Manila hemp is a fibre of remarkable tenacity, of unapproached value for heavy cordage, but too stiff for small cords and twines. After these in utility come Sisal hemp of Central America (Agava Sisalana), Phormium hemp of New Zealand (Phormium tenax) and Sunn hemp of the East Indies (Crotalaria juncea)—all fibres of great strength, and largely used by rope-makers. Jute (q.v.) of India (Corchorus capsularis and C. olitorus) is now largely used by rope-makers on account of its cheapness. When used alone it is deficient in strength and durability, but when used in conjunction with proper proportions of hemp it makes a very satisfactory and useful rope. Among fibres more rarely seen in rope-works are Jubbulpore hemp (Crotalaria tenuifolia), box string hemp (Sanseviera zeylanica), and other hemps of the East Indies, plantain fibre (Musa paradisica), and agave fibre (Agave americana) of America. Coir and many other fibres are used, but principally in the localities of their production.
A rope is composed of a certain number of “strands,” the strand itself being made up of a number of single threads or yarns. Three strands laid or twisted together form a “hawser-laid” rope, and three such hawsers similarly laid make a “cable-laid” rope or “cable.” A “shroud-laid” rope usually consists of four strands laid around a central strand or core. The prepared fibre is twisted or spun to the right hand to form yarn; the required number of yarns receive a left-hand twist to make a strand; three strands twisted to the right make a hawser; and three hawsers twisted to the left form a cable. Thus the twist in each operation is in a different direction from that of the preceding one, and this alternation of direction serves, to some extent, to preserve the parallelism of the fibres.
The primary object of twisting fibres together in a rope is that by mutual friction they may be held together when a strain is applied to the whole. Hard twisting has the further advantage of compacting the fibres and preventing, to some extent, the penetration of moisture when the ropes are exposed to water; but the yield of rope from a given length of yarn diminishes in proportion to the increase of twist. The proper degree of twist given to ropes is generally such that the rope is from three-fourths to two-thirds the length of yarn composing it.
Rope-walk Spinning.—The sequence of operations in this method of working is as follows: (1) hackling the fibre; (2) spinning the yarn; (3) tarring the yarn when necessary; (4) forming the strands; (5) laying the strands into ropes.
Hackling differs but slightly from the hand-hackling process used in the preparation of flax. The hackle board consists of a wooden block studded with strong, tapered and sharp-pointed steel prongs. A series of such hackle boards is used in the progressive hackling operation, the prongs diminishing
