CHAPTER XXII.
NATURAL WOUNDS.
Burrows and excavations. Bark-boring—Wood-boring—Wood fungi—Leaf-miners—Pith flecks—Erosions. Skeleton leaves—Irregular erosions—Shot holes. Frost cracks—Strangulations—Spiral grooving.
Natural wounds are produced in a variety of ways during the life of the plant, and, generally speaking, are easily healed over by the normal process if the area destroyed is not too large, and the parts remaining uninjured are sufficiently provided with foliage, or with supplies of food-materials stored up in the roots, rhizomes, medullary rays, etc., to feed a vigorous callus.
The nature of such wounds and the mode of healing are explained by what we know of artificial wounds, and it only remains to point out that the principal danger of ordinary wounds is not so much the direct traumatic action, because the simpler organisation of the plant does not involve matters connected with shock, loss of blood, etc., as in animals; the danger consists, rather, in their affording access to other injurious agents, especially fungi, and the treatment of wounds frequently resolves itself into cutting or pruning in order to get clean surfaces which can heal readily.
Wounds on leaves imply loss of foliar surface—i.e. of chlorophyll action—and the remarks on [page 193] apply.
Burrows may be taken as comprising all kinds of tunnel-like excavations in the various organs of plants, including those cases where insects burrow into hollow stems of grasses, etc., as indicated by the perforations they make in the outer tissues.
Bark-boring is done by many species of beetles, especially Scolytidae, which excavate characteristically formed branching passages tangentially in the inner bark of Conifers and other trees. Some of them also bore down to the surface of the sap wood (e.g. Tomicus bidentatus) or even burrow right into the latter (e.g. T. lineatum). It commonly happens that the external apertures show up clearly, owing to the brown dust and excrement, sometimes accompanied by turpentine, which exude from them. Many of these Bark beetles only attack trees which are already injured by fire, lightning, etc.; possibly they cannot bore through a cortex which swamps them with sap, as a vigorous one might do.
Wood-boring is also done by many of the bark-beetles as well as by Longicorns, e.g. Saperda in Poplars and Willows, the young shoots of which often show characteristic swellings with lateral holes indicating the points of exit. From the external apertures comminuted wood, like saw-dust, is frequently ejected in quantity and betrays the presence of the insects. Certain wood-wasps (Sirex) and the larvae of moths (Cossus) also make large perforations in the wood of Willows and other trees, often destroying it completely. In the case of these larger borers, whose tunnels may be as broad as the little finger, the foul smell as well as abundant "saw-dust" betray the evil.
Excavations in wood are by no means caused only by insects: several of the larger Hymenomycetes—Stereum, Thelephora, Polyporus, etc.—tunnel the timber in characteristic ways and often after a fashion very suggestive of insects. They usually obtain access through fractures.
Tunnels in leaves are invariably due to the activity of miners belonging to the smaller moths and beetles—e.g. Tinea, Orchestes, etc.—the larvae of which eat out the mesophyll but leave the covering epidermis or cuticle untouched, and since the insect bores forwards only, in an irregular track, and leaves its excrement in the winding passage, the effect is very characteristic.
Whitish leaf tunnels in Peas are excavated by Phytomyza.
Characteristic foxy-red tunnels are mined in the leaves of Apples by Lyonetia, Coleophora, etc.
Falling of fruit, of Apples, Plums, Apricots, etc., before they are ripe, is frequently due to insects, of which the various species of Grapholitha or Carpocapsa are conspicuous: the fallen fruits show a small hole leading by a labyrinth of passages to the "core" or "stone," and in which the grub and its excrement are visible. The cutting off of the vascular bundles and disturbance of the water supply only partly explain the premature fall.
Pith-flecks are minute brown specks or patches found in the wood-layers of many trees, and consist of dead parenchymatous thick-walled cells, reminding one of the structure of pith. They are explained as due to the borings of minute insects, Diptera or Beetles, the larvae of which pierce the cortex and phloem and bore their way into the cambium. The latter then occludes the tunnels by filling them up with cells, and continuing its wood-forming activity gradually buries them deeper and deeper in the wood. Such pith-flecks are common in Willow, Birch, Alder, Sorbus, etc. It is possible that they may be due to other causes also in other trees.
Erosions or irregular wounds on leaves are caused by large numbers of grubs and caterpillars and other insects, such as earwigs, as well as slugs, snails, and other animals; but it must by no means be assumed that all marginal leaf wounds, for instance, are caused by animals, since many fungi which rot the tissues, as explained below ([p. 208]), also cause such erosions, the putrescent parts falling out—e.g. the Potato disease.
Skeleton leaves frequently result from the ravages of caterpillars, which leave the coarser ribs and veins untouched, but much finer skeletons with the minute veins almost intact may be found on plants infested with certain insects—e.g. Selandria on Cherries. Skeletonised patches on Cherry leaves, often pink or brown-pink, are eaten out by this grub.
Shot-holes are perforations in leaves presenting the appearance, from their more or less rounded shape, of gunshot wounds. They may be due to insects which bore through the young leaves while still folded in the bud—e.g. Willow Beetle—or which gnaw out the tissue—e.g. the Beech Miner. Similar but usually more torn and irregular holes are eaten out by many caterpillars—e.g. the Cabbage Moth.
Shot-holes on Peas may be the work of Thrips.
Leaf perforations are commonly caused by severe hail-storms, the hail-stones beating right through the thin mesophyll. Certain chemicals used for spraying have also been known to cause shot-holes by killing the tissue beneath the standing drops.
There is, however, a class of shot-holes in thin leaves which are due to the action of minute fungi, the mycelium of which so rots the tissues in a more or less circular area round the point of infection, that, in wet weather, the decomposing mass falls out and leaves a round hole—e.g. certain Chytridiaceae, Peronosporeae, Gloeosporium, Exoascus, etc. If dry weather supervenes these holes frequently dry at the edges, and the leaves appear as if eaten out.
Shot-holes in Cherry, Walnut, Tobacco, and Plum leaves are due to Phyllosticta, in Cherry leaves also to Clasterosporium, and in Potato leaves to Haltica.
Frost-cracks.—The trunks of trees exposed to the north-east, and occasionally with other aspects, are apt to show longitudinal ridges which realise on a larger scale the features of healed wounds scored with a knife. These wounds are due to the outer layers of wood losing water from their cell-walls as it congeals to ice in their lumina, more rapidly than do the warmer internal parts of the trunk; as this drying of the wood causes its shrinkage, especially in the tangential direction, the effect of a sudden frost and north-east wind is to rend the wood, which splits longitudinally with a loud report, as may often be heard in severe winters. Since the cortex and bark are ruptured at the same time the total effect resembles that of a deep knife-cut, and the same healing processes result on a larger scale when the wood swells and closes up the wound again in spring. But this recently-closed lesion is evidently a plane of weakness, and if a similarly severe winter follows the wound reopens and again heals, and so on, until after a succession of years a prominent Frost-ridge results, which may finally heal completely if milder winters ensue or the tree be eventually protected.
Strangulations.—We are now in a position to understand the so-called strangulations which result when woody climbers, telegraph wires, etc., kill or injure trees by tightly winding round them. If strong wire is twisted horizontally round a stem, the growth in thickness of the latter causes the trapping of the cortex and cambium, etc., between the wire and the wood, and a ringing process is set up in consequence of the death of the compressed tissues. A callus then forms above the wound, as in the case of true ringing by means of a cut, and eventually bulges over the upper side of the wire: in the course of years this overgrowth may completely cover in the wire, and, pressing on to the lower lip of the wound, may at length fuse with the cambium below. Hereafter the thickening rings of wood are continuous over the buried wire. The process is obstructed by all the impediments referred to in dealing with ringing, and of course the stem thickens more above than below the wire. If the sapwood is thin, and the bark is so thick as to put great obstacles in the way of the junction of the upper and lower cambiums, death may result—the tree is permanently ringed. (See [p. 201].)
Spiral grooves are frequently met with where Wood-bine or other woody climbers have twined round a young stem or branch, the upper lip of the groove always protruding more than the lower. If a kink or a crossing of two plants or branches of the twiner results in a complete horizontal ring, the results are as in the above cases of ringing and strangulation. Naturally grooved walking sticks are often seen.
Buried letters, etc.—These processes of healing by occlusion enable us to understand how letters of the alphabet, cut into the wood of trees, come to be buried deep in the timber as successive annual rings cover them in more and more. Chains, nails, rope, etc., have frequently been found thus buried in wood.
Notes to Chapter XXII.
In addition to the [notes] to the last chapter, the reader may be referred to Fisher in Vol. IV. of Schlich's Manual of Forestry, Chap. VI., for an account of Hess' excellent work on Boring Beetles, etc.
The authority on Wood-fungi is Hartig, see especially his Zersetzungs-erscheinungen des Holzes, the principal results of which are condensed in his Diseases of Trees already referred to. As regards "Pith-flecks," the reader should consult Frank, Krankh. der Pflanzen, B. I., p. 212: the subject needs further investigation.
