Description—The capsule which is globose splits into 5 carpels, each containing two seeds separated by a partition. The seeds are of flattened, elongated ovoid form with an acute edge, and a slightly oblique point blunt at one end. They have a brown, glossy, polished surface which under a lens is seen to be marked with extremely fine pits. The hilum occupies a slight hollow in the edge just below the apex. The testa which is not very hard encloses a thin layer of albumen surrounding a pair of large cotyledons having at their pointed extremity a straight embryo. The seeds of different countries vary from ¼ to ⅙ of an inch in length, those produced in warm regions being larger than those grown in cold. We find that 6 seeds of Sicilian linseed, 13 of Black Sea and 17 of Archangel linseed weigh respectively one grain.
When immersed in water, the seeds become surrounded by a thin, slippery, colourless, mucous envelope, which quickly dissolves as a neutral jelly, while the seed slightly swells and loses its polish. The seed when masticated has a mucilaginous oily taste.
Microscopic Structure—On examining the testa under almond oil or oil of turpentine, the outlines of the epidermal cells are not distinctly visible. But under dilute glycerin or in water the epidermis quickly swells up to 3 or 4 times its original thickness; on warming, the entire epidermis is resolved into mucilage, except a thin skeleton of cell-walls, which withstands even the action of caustic lye. The formation of the mucilage may be conveniently studied by the use of a solution of ferrous sulphate, with which thin sections of the testa should be moistened. Other structural peculiarities may be seen if they are imbued with concentrated sulphuric acid, washed and then moistened with a solution of iodine. The application of polarized light is also useful. By the latter means crystalloid granules of albuminoid matter become visible if the sections are examined under oil. The tissue of the albumen and the cotyledons abounds in drops of fatty oil.
Chemical Composition—The constituent of chief importance is the fixed oil which the seed contains to about ⅓ of its weight. The proportion obtained by pressure on a large scale is 20 to 30 per cent. varying with the quality of the seed. The oil when pressed without heat and when fresh has but little colour, is without unpleasant taste, and does not solidify till cooled to -20° C. The commercial oil however is dark yellow, and has a sharp repulsive taste and odour. On exposure to the air, especially after having been heated with oxide of lead, it quickly dries up to a transparent varnish consisting chiefly of Linoxyn, C₃₂H₅₄O₁₁. The crude oil increases in weight 11 to 12 per cent., although at the same time its glycerin is destroyed by oxidation.
By saponification, linseed oil yields glycerin, and 95 per cent. of fatty acids, consisting chiefly of Linoleic Acid, C₁₆H₂₆O₂, accompanied by some oleic, palmitic, and myristic acid. The action of the air transforms linoleic acid into the resinoid Oxylinoleic Acid, C₁₆H₂₆O₅. Linoleic acid appears to be contained in all drying oils, notably in that of poppy seed. It is not homologous either with ordinary fatty acids or with the oleic acid of oil of almonds, C₁₈H₃₄O₂. The chemistry of the drying oils, especially those of linseed and poppy, has been particularly investigated by Mulder.[400]
The viscid mucilage of linseed cannot be filtered till it has been boiled. It contains in the dry state more than 10 per cent. of mineral substances, when freed from which and dried at 110° C. it corresponds, like althæa-mucilage, to the formula C₁₂H₂₀O₁₀. The seeds by exhaustion with cold or warm water afford of it about 15 per cent. By boiling nitric acid it yields crystals of mucic acid; by dilute mineral acids it is broken up into dextrogyre gum and sugar and cellulose.[401]
Linseed contains about 4 per cent. of nitrogen corresponding to about 25 per cent. of protein-substances. After expression of the oil these substances remain in the cake so completely that the latter contains 5 per cent. of nitrogen, and constitutes a very important article for feeding cattle.
In the ripe state linseed is altogether destitute of starch, though this substance is found in the immature seed in the very cells which subsequently yield the mucilage. The latter may be regarded as in analogous cases to be a product of the transformation of starch.
The amount of water retained by the air-dry seed is about 9 per cent.
The mineral constituents of linseed, chiefly phosphates of potassium, magnesium, and calcium, amount on an average to 3 per cent., and pass into the mucilage. By treating thin slices of the testa and its adhering inner membrane with ferrous sulphate, it is seen that this integument is the seat of a small amount of tannin.