Part 17 (1/2)
_Patches--Frost-patches--Bruising due to hail, shot, etc.-- Fire--Sun-burn or scorching--Sun-cracks. Dying-back--Frost-- Fungi--Wound fungi--Defoliation by insects--Defoliation by hand--Staghead._
_Necrosis._--This is a general term for cases where the tissues gradually turn brown or black in patches which die and dry up, the dead area sometimes spreading slowly and invading the usually sharply demarcated healthy tissues around. It is a common phenomenon on the more slender stems or branches of trees, especially those with a thin cortex, and the terms _Brand_ or _Scorching_ sometimes applied signify the recognised resemblance between burnt patches and these dead areas of necrotic tissue.
Necrosis is often due to frost, which kills the cortex of Pears, Beech, etc., in patches of this kind. The dead cortex and cambium stick to the wood beneath and contract as they dry. The living cambium and cortex around them then begin to push in callus towards the centre of the necrotic area; but since this callus is formed under the pressure of the cortical tissues it does not form a thick lip or margin to the healing wound, as it does in a Canker, but insinuates itself with thinned-off edges between the wood and the dead tissue, or at most traps a little of the latter in the final closing up of the wound. It is easy to see how such an area of Necrosis may become a Canker if the dead tissues split or slough off, and fungi or insects obtain access to the callus at the margins of the area, setting up the disturbances described on p. 222. As matter of fact many Cankers--_e.g._ those of the Larch disease, and those due to _Nectria_, or Aphides, etc.--often begin as flattened or depressed areas of Necrosis started by frost, and many small necrotic patches would eventually become Cankers if not healed up by the callus.
Necrosis may also be due to the bruising of the tissues by large hailstones, to gun-shot wounds, or to any form of contusion which kills the living cells of cortex and cambium.
Necrosis is a natural and common result of fire, and it frequently happens after forest-fires which have run rapidly through the dry underwood, fanned by steady winds, that the lower parts of the boles are scorched on one side only. The killed cambium and cortex then dry up in black necrotic patches, which may eventually heal up by intrusion of callus from the uninjured parts.
_Sun-burn_ or _Scorching_.--If thin-barked trees, such as Hornbeam, Beech, Firs, etc., which have been growing in partial shade owing to dense planting, are suddenly isolated by thinning, the impingement of the sun's rays on the south-west side during the hottest part of summer days may kill the cambium, and produce necrosis of the cortical tissues, and such necrotic patches heal very slowly or not at all, because the dead tissues have contracted so tightly on to the wood below that the callus cannot readily creep between.
_Sun-cracks_ are due to intense insolation on the south side of trees in clear weather in early spring, causing the drying and contraction of the wood and its coverings down that side of the tree: the contracted tissues consequently split, as in the case of frost-cracks, the healing up of which is very similar.
_Dying-back._--All that is true of the necrosis of cortical tissues in small patches also applies to cases where the whole of the outer tissues of thin twigs and branches die of inanition owing to a premature fall of leaves--_e.g._ after a severe attack of some insect or fungus pest. The consequent arrest of the transpiration current and the proper supply of nutriment to the cambium and cortex explain the phenomena. The younger branches of Coffee trees suffering from severe attacks of leaf-disease are often denuded of leaves and die back from the causes mentioned, the whole of the outer tissues becoming necrotic, and drying up tight on to the wood, because other branches with functionally active leaves on them divert the transpiration current, and drought and inanition supervene.
Dying-back is frequently also a direct effect of early frosts, which kill the thin twigs before the ”wood is ripened,” as gardeners say.
Dying-back is also a frequent result of direct frost action on thin watery shoots or ”unripe wood,” and is apt to occur every year in certain varieties of Roses, for instance, in particular situations, such as ”frost-beds,” or aspects exposed to cutting winds, and so forth. The necrosis which results may affect all the tissues, or only the cortex and cambium, and the frequent accompaniment of all kinds of saprophytic _Ascomycetes_ and moulds or other fungi is in no way causal to the phenomenon.
Dying-back may also be caused by fungi, and not necessarily parasites, for cases are often observed where saprophytes only are to be found in the necrotic tissues of the cortex, having made their way in through minute cracks, lenticels, etc.
A simple case is often seen in Chrysanthemums, Roses, etc., chilled and wetted to danger point, but not frozen, during the nights of autumn. The lowered resistance of the chilled tissues enables fungi like _Botrytis cinerea_ to gain a hold, and the peduncles die-back with all the symptoms of Necrosis, the fungus gaining power more and more as its mycelium spreads in the dead tissues.
Many other cases are known where wound-fungi, such as _Nectria_, _Cucurbitaria_, _Phoma_, etc., in themselves incapable of true parasitism, gain a hold on the necrotic tissue of a wounded twig, and having laboriously acc.u.mulated a vigorous mycelium saprophytically, extend into other parts. In many of these cases the dying-back of the twigs is expedited owing to the mycelium invading the medullary rays and wood vessels, and so obstructing the transpiration current. The much more rapid spread of the hyphae up into the parts thus killed sufficiently indicates the fundamentally saprophytic character of such fungi.
Dying-back in all its forms is a common result of defoliation by insects, _e.g._ caterpillars, especially if it occurs when the wood is depleted of reserve materials, and thus cannot supply the auxiliary buds and enable the twigs to clothe themselves with a new flush of foliage, a common danger in Conifers.
Any form of defoliation--_e.g._ excessive plucking of tea and mulberry leaves, browsing of animals, etc.--exposes the twigs to the dangers of dying-back, the accessory phenomena being similar to those already described.
_Stag-head._--Old trees, though vigorous and in full foliage throughout the crown generally, frequently lose the power of bearing leaves on their topmost branches and twigs, which stand out bare and brown, and fancifully resemble the antlers of a stag: hence the forester's name ”stag-head.” This ”top-dry” condition is frequently due to the removal of litter, or to excessive draining, or to the roots having gradually penetrated into unsuitable soil. The consequence is that some dry summer the drought causes the breakage of the water columns above, and the twigs die back.
Tropical trees may also become _stag-headed_ owing to the attacks of _Loranthus_ and other parasites, the portions above the point of attachment dying back from inanition.
Cases also occur in the tropics where the _stag-head_ condition is due to the persistent roosting of frugiferous bats--”flying foxes”--which tear the bark and foliage with their claws, and befoul the twigs generally.
NOTES TO CHAPTER XXVI.
The princ.i.p.al literature as regards frost is given in the works of Frank, Sorauer, and Hartig already referred to. An excellent summary will be found in Hartig's _Diseases of Trees_, p. 282, and in Fisher ”Forest Protection,” Vol. IV. or Schlich's _Manual_, p. 423.
CHAPTER XXVII.
MONSTROSITIES AND MALFORMATIONS.
_Monstrosities--Teratology--Atrophy of organs--Shanking of grapes--Barren fruit trees--Dwarfing--Distortions and malformations--Fasciations--Flattened roots--Torsions--Curling and puckering--Leaf rolling--So-called ”spontaneous”
teratological changes._
_Monstrosities._--In a wide sense this term is applicable to many cases here treated under other headings, and signifies any departure from the normal standard of size, form, arrangement, or number of parts, and so forth, due to arrest of growth, excessive growth of parts, or of the whole organs, etc.