In a vast majority of cases the secondary internal membrane of some of the ducts is perforated by orifices of numerous forms, sometimes irregularly, and sometimes in a regular pattern like a sieve, and on that account they are called the pitted tissue.
In the stem of a tree the vascular cylinders alternate with cylinders of woody fibre, as may be seen in a section perpendicular to the axis, in which the two tissues form a series of alternate rings. The woody or ligneous tissue, which gives strength and solidity to all vegetable structures, consists of bundles of nearly parallel spindle-shaped tubular fibres, having their attenuated extremities applied end to end to the extremities of those above and below them, so that they form groups of nearly straight lines; but although the ends of these tubular fibres overlap each other they do not prevent a free circulation of the sap. The different layers of these combined tissues which form the wood do not convey the rising sap in equal quantity. The outermost layers that are nearest the bark, which are always the last formed or youngest, convey it in greatest quantity, and on that account are called the sap-wood: the older the layers the less they convey, because the interior walls of the cells of both tissues are coated with successive layers of a mucilaginous substance which is the colouring matter of the wood (lignin), and is called sclerogen, which becomes hard, is ultimately united to the cell walls, and fills or nearly fills the tubular fibres and vascular ducts, so that those nearest the centre of the tree lose or nearly lose the power of conducting the sap, as in hard wood like the oak, though in softer wood, as the lime tree, it is not entirely lost. Ligneous tissue forms the chief part of the stems, branches, and shoots of trees and shrubs; it gives firmness to leaves, flowers, and all their parts, and strength to the stems and skins of herbaceous plants; it is found in the bark of all trees, and constitutes the strong fibre of hemp, flax, the agave, and many other plants, whence linen, canvas, and cordage are made. Cells lined with sclerogen form the shells of nuts, cocoa-nuts, and walnuts, as well as cherry, peach, and plum stones, the brown coat of apple and pear seeds, the gritty particles in the heart of the pear, the white coats of the pips of the orange and lemon, the husks of peas, &c.