1076. The effort of the three vegetable processes, to separate their organs from, and perfect them independently of, each other, is in incessant operation, both from without inwards, as well as from above downwards, because in both cases is light there, darkness here, dryness there, humidity here. The cellular tissue, that has finally become independent in the transverse direction or from without inwards, is called _bark_, the self-substantial tubular tissue is called _liber_, the non-dependent tracheal tissue, _wood_. 1. TRACHEAL-SYSTEM, WOOD. 1077. With increased influence of light the tracheal fasciculi also increase, and form a circle of columns in the parenchyma around the centre of the plant. Between the column, externally and internally to the same, is the parenchyma. The more, however, the columns accumulate, by so much the more does it diminish, and whereas the columns previously stood singly in the parenchyma, the appearance is now as if narrow _plates_ only of parenchyma traversed between the columns from without inwards. Finally the columns predominate to such an extent and approximate so closely, that the _plates_ almost disappear. They are now called insertions of cellular tissue, or _medullary rays_. As the tracheæ convey air, and have thus been more exposed to the process of oxydation, they generally harden sooner than in other parts. 1078. Around the fasciculi of spiral vessels the cellular tissue also strives to elongate, and begins at the same time to harden. Such extended cells, in which the light has almost disappeared, are called fibres. Indurated tracheæ and fibres are called _Wood_. The wood is always in the vicinity of the spiral vessels. It is a production synchronous with the latter. 1079. Only, where spiral vessels are, can genuine wood originate; but it is not everywhere, where they are found, that woody fibres must be also present, although the cells extend around all bundles of spiral vessels. If the degree of oxydation of the cells be slight, they do not harden, but continue herbaceous in texture. The parenchyma has now been separated by a circle of fibrous columns into an external and internal, or peripheric and central. The central parenchyma becomes void of sap and spongy, because the plant imbibing its nutriment on the surface, and the air and the light operating thereupon, the processes conduct it thither. This withered parenchyma is called _pith_, which in accordance with its origin merits no physiological consideration, nor is worthy and susceptible of any philosophical construction. 2. TUBULAR SYSTEM, LIBER. 1080. As the plant draws in its nourishment from without, so is the main proportion of the sap necessarily present in the periphery of the spiral vessels. The elongated cells in the neighbourhood of the spiral vessels, and which principally contain sap, are called _Liber_.