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.