. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. 36 THE VEGETABLE CELL IN addition of water, this cavitj- becomes more clearly defined, the whole mass of the cell swells, and the mucilage can then be made out as a distinctly stratified strnctnre belong- ing apparently as much to the outer as to the inner face of the cell-wall. But if the action of water is prolonged, the stratified ap- pearance vanishes, and the wall becomes optically ho- mogeneous, with the excep- tion of its middle portion, the so
. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. 36 THE VEGETABLE CELL IN addition of water, this cavitj- becomes more clearly defined, the whole mass of the cell swells, and the mucilage can then be made out as a distinctly stratified strnctnre belong- ing apparently as much to the outer as to the inner face of the cell-wall. But if the action of water is prolonged, the stratified ap- pearance vanishes, and the wall becomes optically ho- mogeneous, with the excep- tion of its middle portion, the so called primary mem- brane, which remains un- changed. On the addition of iodine and sulphuric acid, the primary membrane, but not the mucilage, becomes blue. Furthermore, the lateral walls of the cells are not converted into mucilage. 150. The mucilaginous modification can be examined to ad- vantage in the seeds of some Polemoniacese (especially Collomia) and a few Acanthaceae, e. g.^ Ruellia. These seed-coats are covered with hairs which breali open when wet, and allow not only the mucilage but also slender coiled threads to escape. The achenes of some Compositse of the Senecio group and the nutlets of a few Labiatffi (the Salvia tribe) exhibit nearly the same phenomenon. 151. Lig'nifleation. Induration of the cell-wall is caused most eommonlj' by the presence of an incrusting substance known as lignin. Owing to the difficult}- of separating it from the cellulose, with which it is associated, its chemical composi- tion must be regarded as uncertain. Although general!}' spoken of as a single substance, it is probable that the lignin, or in- crusting matter, is made up of several different substances,^ ' Payen (Mem. des savants Strangers, ix., 1846, (ip. 68, 5) distinguished four such incrustiug matters, ditt'ering in their compoaitiou and in their be- havior to solvents. Lignose: insoluble in water, alcohol, ether, and ammo- nia; soluble in solutions of potassa and soda. Lign
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