An ordinary cambium is scarcely ever found in the Monocotyledons, but in certain woody forms a secondary meristem is formed outside the primary bundles, and gives rise externally to a little secondary cortex, and internally to a secondary parenchyma in which are developed numerous zones of additional bundles, usually of concentric structure, with phloem surrounded by xylem.
By the former method the rods are left on the ground until spring advances, when a rapid growth of the cork cambium begins.
The activity of the new cambium is often associated with the stoppage of the original one.
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.
In many annual plants no cambium is formed at all, and the same is true of most perennial Pteridophytes and Monocotyledons.
In the simplest cases the cambium produces xylem more freely along certain tracts of the circumference than along others, so that the stem loses its original cylindrical form and becomes elliptical or lobed in section.
The fungus-mycelium will go on growing indefinitely in the cambium layer, thus killing and destroying a larger area year by year.
Aphidesand may be easily penetrated by certain Fungi such as Peziza, Nectria; and when thus attacked, the repeated conflicts between the cambium and callus, on the one hand, trying to heal over the wound, and the insect or Fungus, on the other, destroying the new tissues as they are formed, results in irregular growths; the still uninjured cambium area goes on thickening the branch, the dead parts, of course, remain unthickened, and the portion in which the Fungus is at work may for the time being grow more rapidly.
Irregularity of cambium occurs in various families of woody dicotyledonous plants, mostly among the woody climbers, known as lianes, characteristic of tropical and sub-tropical forests.
At celtain points the cambium does not give rise to xylem and phloem elements, but cuts off cells on both sides which elongate radially and divide by horizontal walls.