Part 14 (2/2)

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 princ.i.p.al 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.

CHAPTER XXIII.

EXCRESCENCES.

_Herbaceous excrescences, or galls--Erineum--Intumescences-- Corky warts, etc.--Pustules--Frost-blisters--Galls and Cecidia --Root nodules._

_Excrescences_, or out-growths of more or less abnormal character from the general surface of diseased organs, are very common symptoms, and widely recognised. They are due to hypertrophy of the tissues while the cells are young and capable of growth, and may be induced by a variety of causes, among which the stimulus of insect-punctures and of the presence of insect eggs are best known; but that of fungi, though less widely recognised, plays an equally important part, and, as we shall see, galls and other excrescences may be due to widely different agents.

_Galls_ or _Cecidia_ are protuberances of the most varied shapes, colours, and sizes found on herbaceous parts attacked by insects, fungi, etc. In the simplest cases the insects only pierce and suck the young cellular tissue--_e.g._ _Phytoptus_, Aphides, etc.--but in others the stimulus to hypertrophy starts by the puncture of the embryonic tissue of a leaf, root, etc., by the ovipositor of the female insect, which then lays an egg--_e.g._ _Cynips_, _Cecidomyia_, etc.--the presence of which appears to intensify the irritating action, or such only occurs when the young larva escapes.

Our knowledge of the primary cause of gall-formation amounts to very little. Generally speaking, only embryonic or very young cellular tissue reacts, and galls on adult leaves and branches have usually been initiated long before. The same gall-insect may induce totally different galls on different plants, or even on different parts of the same plant, and different insects call forth different galls on any one plant. These facts point clearly to the co-operation of both plant and insect in the gall-formation, and the best hypothesis yet to hand is to the effect that a gall is a hypertrophy of cells, the normal nutrition, growth, and division of which have been disturbed owing to the action of some poison or other irritant derived from the insect, or fungus, or other organism.

Attempts have been made to reproduce galls by injecting the juices of similar galls into the tissue, but as yet without success, and this may point to the co-operation of mechanical irritation during the hypertrophy in normal gall-formation.

Galls, in the broad sense, are not always preceded by a wound, however.

Insects on the outside of young tissues may cause such irritations that the parts in contact with the animal are arrested in their growth, while those further away grow more rapidly--_e.g._ where Mites, etc., cause puckers and leaf-rolling. In true galls the hypertrophy may consist merely in the enlargement of cells already present, and no new cell-divisions and, still less, changes in the nature of the tissues result--_e.g._ some pocket galls on _Viburnum_, _Pyrus_, etc., and the hairy outgrowths of the epidermis known as _Erineum_. In other cases there is not only hypertrophy of existing cells, but new cell-divisions are inst.i.tuted: these cell-divisions may be confined to the direction perpendicular to the epidermis, and the tissues grow only in the direction of the surface, producing puckerings--_e.g._ the Aphis galls on _Ribes_, Phytoptus galls of _Salvia_, leaf galls on _Tilia_, _Acer_, _Alnus_, etc., and the curious galls on Plums due to _Cecidomyia Pruni_, and which must not be confounded with the ”pocket plums” and similar galls due to Exoasci.

In a third series of cases, cell-divisions occur parallel to the surface of the leaf, and galls are formed which grow in thickness, and develop the most extraordinary and complicated new tissues--proteid-cells surrounding the egg or larva deposited inside, followed by a protective layer of sclerenchyma encasing this food layer, and around this again softer tissues which may a.s.sume the structures and functions of respiratory tissues, water-storing tissues, starch reservoirs, a.s.similatory, or protective tissues of various kinds, and over all may be a well-marked epidermis, with stomata, or cork with lenticels.

The chief seat of these hypertrophies and--what is more remarkable--development of new tissue elements not found elsewhere in the leaves, or even in the species, is the mesophyll, and various speculations and hypothesis have been founded on these curious phenomena.

_Erineum._--The simplest excrescences on plants are certain hair-like developments of epidermal cells due to the irritation of species of _Phytoptus_, and similar insects which rise in cl.u.s.ters on the surfaces of leaves and by their colours, consistence, arrangement in patches, spots, etc., so simulate fungi that Persoon was deceived by them and gave them the genus name _Erineum_. They occur on most of our trees, _e.g._ Poplar, Lime, Oak, and are very common in the Tropics. Usually pale or even white at first, they turn brown as the hair-like outgrowths die and lose their sap, but since the latter may be bright coloured--yellow, red, purple,--the patches are sometimes very conspicuous objects on smooth leaves.

In many cases these hairs exactly resemble in shape and other characters the abnormal root-hairs found on roots exposed to the effects of poisonous reagents, or of unsuitable food-materials, or the rhizoids developed from wounded Algae, etc.

_Intumescences_ are similar trichomatous outgrowths not a.s.sociated with insects or fungi, and due to some disturbance of the balance between transpiratory and a.s.similatory functions of their leaves, as indicated by the less localised occurrence and by their non-appearance when the plant is under favourable cultural conditions. Structures not unlike these have been artificially induced by exposure to particular lights, and also by painting spots with dilute corrosive sublimate, indicating that poisons may impel the epidermis cells to grow out abnormally.

_Corky warts._--Several forms of disease are known in which the pathological condition is expressed by the formation of cork in unwonted places and quant.i.ties. The _Scab_ or _Scurf_ of Potatoes is a case in point. The tissue of the lenticels absorbs water and the outermost cells are cut off by cork and die: the cells below them burst the dead bark-like ma.s.ses thus formed, and again cork is formed and cuts off the outer ma.s.ses, and the rough cork warts--_Scab_ or _Scurf_--are the result.

The causes predisposing to scab have been variously a.s.signed to dampness, want of lime, action of bacteria and fungi--_e.g._ _Sorosporium_, _Oospora_, _Spongospora_,--the latter making their way into the ruptured tissue of the lenticels and irritating the cells to further growth.

It seems probable that several different kinds of scab exist in Potatoes, as well as in roots--_e.g._ Beets, and the whole subject needs further investigation. The scab-like rough scaly bark of Pear trees in dry districts may also be mentioned here.

_Cork-wings_ are well known on the young branches of Elms, Maples, etc., some varieties of which have received specific names on this account.

_Corky excrescences_ on leaves occur occasionally in the Gooseberry, Holly and other plants, for which no cause has been discovered.

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