beech, birch and maple, being difficult to treat, and yielding a pulp of inferior quality and colour.

The cells or elements which form the woody tissue of plants are varied in shape and size. The complex structure of the plant stem is due to the association of true woody fibres with tracheids, or pitted cells, and medullary tissue. In the Coniferæ, or cone-bearing trees, the wood is made up entirely of tracheids,

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Fig. 114.-Spruce Wood Fibre, according to Herzberg.

which cells are easily recognised in spruce. With poplar, the elements are true woody fibres, or libriform cells, which present an entirely different appearance.

Chemical Wood Pulp. The treatment to which wood is submitted for the preparation of chemical pulp isolates the cells from one another in such a manner as to give them their maximum length and true form. The markings on the cells vary in character according to the particular wood, so that it is possible, by careful observation of the cells under the microscope, to identify the wood from which the pulp has been made.

Spruce. The fibres of spruce are most easily recognised by their characteristic flat ribbon-like shape and the existence of circular pitted vessels, or pores, in the fibres. The circular markings are usually prominent in hard strong sulphites, but they are often less distinct in well-boiled pulps.

Occasionally cells assume the twisted shape peculiar to cotton, but they can be differentiated by the zinc chloride and iodine solution, which imparts a bluish colour, whereas cotton is coloured brown.

The shape and distribution of the pores in the fibres afford some indication of the particular tree employed. For example, Pinus picea is characterised by minute pores grouped in series of four, and Pinus sylvestris by large egg-shaped pores distributed along the fibres at fairly regular intervals.

Poplar. The pulp of poplar and similar non-resinous woods is different from that of the conifers spruce and pine. These fibres are shorter and more cylindrical in shape and frequently characterised by knots in the cell wall which give them the appearance of linen. In addition to the true fibres the pulp contains a large proportion of curiously shaped cells, the markings of which are most distinctive. Birch contains a greater number of such cells than poplar.

Birch.-Birch fibres somewhat resemble straw, more particularly those straw

fibres having thick walls. The pores are small and oval-shaped. The ends of the fibres are too variable in shape to be considered distinctive.

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Fig. 146.-Birch Wood Fibre (Herzberg).

The cells are very characteristic, being large, broad, and pointed, marked with narrow pores lying across the cells in great number.

Mechanical Wood Pulp.-The drastic treatment in the mechanical process by which the pulp is manufactured results in the formation of a mass of fibres and fibrous elements possessing little "felting" power. The pulp consists of a mixture of structureless particles and fibres still preserving the characteristic

Fig. 147.-Mechanical Wood Pulp, according to Herzberg. 1, 2, Well-formed fibres, showing pitted vessels: 3, Bundles of fibres, united by medullary tissue; 4, 5, Structureless particles; 6, Typical wood cell (a) surface view, (b) side view

features which serve for identification. The structure of the latter can be observed, and the exact form of the cells, pitted vessels, and medullary rays determined, so that the particular class of wood from which the pulp has been prepared can generally be distinguished.

The mechanical wood is not only easily recognised by the structure, but is more readily detected by the deep yellow colour imparted to it by iodine, or by zinc chloride and iodine solution. Other lignified fibres such as jute are coloured in a similar manner, but cannot be confused with ground wood on account of the great difference in structure.

Other Fibres.--Many other fibres are suitable for the manufacture of paper, but up to the present time the utilization of plants other than those

1, 2, Bast cells; 3, Point of a bast cell; 4, 5, Cuticular cells; 6, Pithy matter.

already described has never been carried much beyond the experimental stage. Amongst them may be mentioned:

Adansonia. This is the inner bark of the Baobab tree (Adansonia digitata). It is capable of giving a strong paper and was tried some few years ago in England.

Agave. The Aloe plant, used mainly for cordage. Fibres long and stiff. Bagasse.-The crushed stalk of the sugar-cane. Easily converted into pulp, but giving a low yield of pulp. Can only be handled profitably where grown.

Bamboo (Bambusa vulgaris).—This plant gives excellent pulp, as shown by Mr. Thomas Routledge in 1875. The pulp resembles straw in some respects, consisting of thick walled bast fibres and numerous serrated and ovcid cells.

China grass (Rhea, Boehmeria nivea). A shrub which yields bast fibres from the bark. The ultimate fibre is long, in some cases reaching to eight inches.

Cocoanut fibre (Cocos nucifera).—Obtained from the cocoanut husk. Merely of experimental interest.

Munj. A grass largely used in Indian paper-mills.

New Zealand flax (Phormium tenax).-A strong fibre, but little used. Paper mulberry (Broussonetia papyrifera).—A fibre largely used in Burma, Japan and China, specially suited for the papers characteristic of Japan, &c. Peat.--Used in limited quantities for cheap common brown papers.

Sunn hemp (Crotolaria juncea).-A strong fibre used in India for the manufacture of rope and matting.