The fibro-vascular bundles which constitute the wood of trees, and form consecutive cylinders round the stem between the pith and the bark, consist of vascular ducts and woody fibre. The vascular ducts are formed of large wide cells each standing upon the other’s flattened end, their cavities being thus separated from each other by septa or partitions directed at right angles to their longitudinal axis. This vascular tissue when young conveys the sap from the roots through the stem and branches to the leaves. It forms part of the stems of all climbing and quick-growing plants, in which the circulation of the sap is rapid, and the perspiration great. The sap in this crude state passes freely through the partitions, being probably a dialysable liquid; but in the autumn, when the sap ceases to rise, the septa are either absorbed or destroyed, as appears from the fragments of them that sometimes remain, and then the ducts become filled with air, which they convey to mature the sap in the leaves and all parts of the plant. Some of the vascular ducts have very narrow parallel fibres of a bluish colour twisted in a more or less elastic spiral from end to end of their internal surface, which in by far the greater number of cases turns in the same direction as a left-handed screw. In some ducts they merely cross the inner wall of the cell at regular distances as circles. The reticulated form from the crossing of right and left-handed spirals is still more frequent than the simple spiral; there is scarcely a plant, from the mosses upwards, in which that structure cannot be found. 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.