Some plants root so freely that they need only pegging down; but in most cases the arrest of the returning sap to form a callus, and ultimately young roots, must be brought about artificially, either by twisting the branch, by splitting it, by girding FIG.
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.
A canker is the result of repeated frustrated attempts on, the part of the callus to heal up a wound.
It is from the actively growing callus developed at the surface of the wounded tissues of cuttings, buddings, prunings, &c., that the healing and .renewal of tissues occur of which advantage is taken in the practice of what might well be termed plant surgery.
Wounds.The principal phenomena resulting from a simple wound, and the response of the irritated c~lls in healing by cork and in the formation of callus, have been indicated abeve.
Bruising by a cartwheel, insects-having started a callus on which adventitious buds arise, or to the destruction of buds at an early stage.
If a clean cut remains clean, the cambium and cortical tissues soon form callus over it, and in this callusregenerative tissuenew wood, &c., soon forms, and if the wound was a small one, no trace is visible after a few years.
But the occluding callus is a mass of delicate succulent cells, and offers a dainty morsel to certain insects e.g.
Sieve Tubes.The sieve tubes consist of partially fused rows of cells, the transverse cr lateral walls being perforated by minute openings, through which the contents of the cells are connected with each other, and which after a certain time become closed by,the formation of callus on the sieve plates.
It frequently extends downwards a little on the rachilla, forming with the latter a swollen callus, which is separated from the free portion by a furrow.
She'd built up a callus around her heart, one that got stronger and easier to maintain as she manipulated evil men.
She'd buried them under the callus she'd grown around her emotions.
Frost blisters are pustular swellings due to the up-growth of callus-tissue into cavities caused by the uprising of the superficial cortex under the action of intense cold.
Such frost-cracks, sun-cracks, &c., may then be slowly healed over by callus, but if the conditions for necrosis recur the crack may be again opened, or if Fungi, &c., interfere with occlusion, the healing is prevented; in such cases the local necrosis may give rise to cankers.