According to the relative positions of the xylem and phloem elements, there are two principal kinds of conjoint bundles—the collateral and the concentric. Of these again there are three varieties, but the experiments with leaves bring out parallel facts; that in ordinary stems the staining of the wood by an ascending coloured liquid is due, not to the passage of the coloured liquid up the substance of the wood, but to the permeability of its ducts and such of its pitted cells as are united into regular canals; and the facts showing this at the same time indicate with tolerable clearness the process by which wood is formed, for what in these cases is seen to take place with dye may be fairly presumed to take place with sap.
Taking it, then, as a fact that the vessels and ducts are the channels through which the sap is distributed, the varying permeability of their walls, and consequent formation of wood, is due to the exposure of the plant to intermittent mechanical strains, actual or potential, or both, in this way. If a trunk, a bough, shoot, or a petiole is bent by a gust of wind, the substance of its convex side is subject to longitudinal tension, the substance of its concave side being at the same time compressed. This is the primary mechanical effect. The secondary is when the tissues of the convex side are stretched, and also produce lateral compression. In short, the formation of wood is dependent upon transverse strains, such as are produced in the aerial parts of upright plants by the action of the wind.
Fig. 316.—Termination of Vascular System.
1.—Absorbent organ from the leaf of Euphorbia neriifolia. The cluster of fibrous cells forming one of the terminations of the vascular system is here embedded in a solid parenchyma.
2.—A structure of analogous kind from the leaf of Ficus elastica. Here the expanded terminations of the vessels are embedded in the network parenchyma, the cells of which unite to form envelopes for them.
3.—End view of an absorbent organ from the root of a turnip. It is taken from the outermost layer of vessels. Its funnel-shaped interior is drawn as it presents itself when looked at from the outside of this layer, its narrow end being directed towards the centre of the turnip.
4.—Shows on a larger scale one of these absorbents from the leaf of Panax Lessonii. In this figure is clearly seen the way in which the cells of the network parenchyma unite into a closely-fitting case for the spiral cells.
5.—A less-developed absorbent, showing its approximate connection with a duct. In their simplest forms these structures consist of only two fenestrated cells, with their ends bent round so as to meet. Such types occur in the central mass of the turnip, where the vascular system is relatively imperfect. Besides the comparatively regular forms of these absorbents, there are forms composed of amorphous masses of fenestrated cells. It should be added that both the regular and irregular kinds are very variable in their numbers: in some turnips they are abundant, and in others scarcely to be found. Possibly their presence depends on the age of the turnip.
6.—Represents a much more massive absorbent from the same leaf, the surrounding tissues being omitted.