New tangential walls arise in the cells which are the seat of cambial activity, and an initial layer of cells is established which cuts off tissue mother-cells on the inside and outside, alternately contributing to the xylem and to the phloem.
In a good many cases, sometimes in isolated genera or species, sometimes characteristic of whole families, so-called anomalous cambial layers are formed in the stem, either as an extension of, or in addition to, the original cambial cylinder.
If this division occurs by means of a localized secondary meristem connecting the cambial layers of adjacent bundles, an inlerfascicular is formed in addition to the fascicular cambium.
Sometimes the original cambial ring is broken into several arcs, each of which is completed into an independent circle, so that several independent secondary vascular cylinders are formed.
If the development of secondary tissues is to proceed further, arcs of cambium are formed in the pericycle external to the primary xylems, and the two sets of cambial arcs join, forming a conti,riuous, wavy line on transverse section, with bays opposite the primary phloems and promontories opposite the primary xylems. Owing to the resistance offered by the hard first-formed secondary xylem, the bays are pushed outwards as growth proceeds, and the wavy line becomes a circle.
Secondary xylem and phloem produced by a single cambium, or by successive cambial zones; no true vessels (except in the Gnetales) in the wood, and no companioncells in the phloem.
The formation of additional cambial cylinders or bands occurs in the most various families of Dicotyledons and in some Gymnosperms. They may arise in the pericycle or endocycle of the stele, in the cortex of the stem, or in the parenchyma of the secondary xylem or phloem.