Lenticels are also formed on some leaf-galls, and are remarkable as being structures not normal on leaves.

_Pustules._--This term may be employed generally for all slight upheavals of the surfaces of herbaceous organs, which subsequently burst and give egress to the spores, etc., of the organism causing them, or merely fray away at the top if no organism is discoverable. They are often due to fungi--_e.g._ _Synchytrium_, _Protomyces_, _Cystopus_, and Ustilagineae,--and we may extend the use of the general term also to those cases where the _stroma_ of the fungus itself bursts through the cortex of older parts and forms the princ.i.p.al part of the pustule--_e.g._ _Monilia_, forming white or grey pustules on Apples, _Roestelia_ and other aecidia, forming yellow or orange pustules on leaves, etc.; _Cucurbitaria_ and _Nectria_ (red) breaking through the cortex of trees, and _Phoma_ and numerous other Ascomycetes which form black cushions. _Pustules_ on the leaves of _Lysimachia_, _Ajuga_, etc., are due to the parasitic Alga _Phyllobium_.

Cylindrical stem swellings are caused by _Calyptospora_: they are due to the hypertrophy of the cortex of Bilberry stems permeated by the hyphae. _Epichloe_, which clothes the sheaths and halms of gra.s.ses with its stroma, at first snowy white and later ochre-yellow as the perithecia form, is another example.

The cylindrical layer of eggs of a moth such as _Bombyx_ on a twig must not be confounded with these cases.

_Frost-blisters_ are pustule-like uprisings of the cortex, where the living tissues below have formed a callus-like cushion into the cavity beneath the dead outer parts of the cortex which were killed by the frost; they occur on the stems of young Apples, Pears, etc.

_Galls_ in the narrower sense are tissue outgrowths usually involving deeper cell-layers. They are so varied and numerous that cla.s.sification is difficult. For symptomatic purposes we may divide them as follows:

_Leaf-galls._--A well-marked type is that of the _pocket-galls_ or _bladders_ in which the whole thickness of the leaf is as it were pushed up like a glove-finger at one spot, so that if the upper surface of the leaf forms the outside of the gall the lower surface is its lining. Such galls are common on Limes (_Phytoptus_), _Glechoma_ (_Cecidomyia_), Elms (_Tetraneura_), etc. Similar localised extension of the leaf surface, compelling it to rise up like a pocket, are caused by fungi--_e.g._ _Taphrina_ on Poplars, _Exoascus_ on Birches, etc., _Exobasidium_ on Bilberries, Rhododendrons, etc.

Another type is that of the _Gall-apple_, so well known on Oaks, where the spherical swelling is solid--except for the inner cavity containing the eggs--_Neurotus_, _Cynips_, _Hormomyia_, etc. These are comparable in general characters to the nodules on roots.

Fungus galls with similar external features when young are found on Maize (_Ustilago Maydis_), and betray their nature by the black powdery spores as they mature.

Bud galls on Willows are due to _Cecidomyia_, which causes several internodes to swell out into a greenish barrel-shaped ma.s.s, from which leaves may spring.

Small irregular excrescences on Willow stems are referred to _Phytoptus_, and another species of the same insect induces similar swellings on Pines which are not surcharged with resin.

_American Blight_, or Woolly Aphis, on Apples especially, causes the tumour-like swellings covered with sticky white fluff, which is a waxy excretion of the insect. Galls on _Pilea_, in Java, are due to an Alga--_Phytophysa_.

_Root-nodules_ or _nodosities_ are frequently caused by insects--_e.g._ _Centhorhynchus_, a beetle which attacks Crucifers, _Cynips_ and allied "gallflies" of Oaks, and the notorious _Phylloxera_. But similar root-galls are produced by Nematode worms, _Heterodora_, on Beets, Tomatoes, Cuc.u.mbers and numerous other plants, and by the Slime fungus _Plasmodiophora_, and it is not always easy to distinguish such cases from the fungus-galls (_Mycocecidia_) on the roots of Alders, _Juncus_, and Leguminoseae where the symbiosis of bacteria or fungi with the roots are of benefit to the plant. _Urocystis Leimbachii_ forms similar nodules at the collar of young plants of _Adonis_.

_Heterodora javanica_ pa.s.ses into the cortex of sugar-cane roots through fissures, and makes its way to the place where a young rootlet is about to emerge; here it sticks its beak into the growing-point and remains fixed.

Molliard has shown that in the roots of Melons, _Coleus_, etc., _Heterodora_ causes the cells in immediate contact with its head, and which would normally become vessels of the xylem, to swell up into huge giant-cells, with their walls curiously folded, and containing large supplies of proteids and numerous nuclei, reminding us of the food-layer of insect galls and of the tapetal layer of pollen-sacs. While the stimulus exerted by the Nematode thus induces hypertrophy and storage with food-substances of these cells, those of the next layers undergo reticulate thickenings of their walls. Again instances of the evolution of new tissue elements by the action of the foreign organism.

So far as galls on leaves are concerned the amount and kind of damage done are in proportion to the area of chlorophyll action put out of play for the benefit of the plant, and the remarks already made on p. 193 apply here also. Where buds are destroyed the effects may of course extend further, but it rarely happens that leaf-galls are so abundant as to maim a tree permanently. Nevertheless we must remember that cases like _Phylloxera_ are notorious.

Far more dangerous, however, are the root-galls due to such insects, because here the damage is not so local: the water-supplies are cut off, and injurious consequences result from the absorption of the products of decomposition in the soil.

NOTES TO CHAPTER XXIII.

In addition to the literature on galls quoted in the Notes to Chapter XIV., the reader should consult Dale "On certain Outgrowths (Intumescences) on the green parts of Hibiscus,"

_Proc. Cambr. Phil. Soc._, Vol. X., 1899, p. 192, and _Brit.

a.s.s. Rep._, Bradford, 1900.

The detailed study of the anatomy and histology of Galls has been recently undertaken by Kuster, "_Beitrage zur Kenntniss der Gallenanatomie_," Flora, B. 87, 1900, p. 117, where the princ.i.p.al references will be found.

On the root-galls due to Nematodes see Atkinson in _Science Contributions from the Agric. Expt. Station, Alabama_, Vol.

I., p. 1, 1889; Percival, "An Eel-worm disease of Hops" in _Natural Science_, Vol. VI., 1895, p. 187; and Molliard in _Revue generale de Botanique_, Apl., 1900, p. 157, where the histology is dealt with.

The nodules of the roots of Leguminoseae are not part of the subject of this work: the literature is collected in _Science Progress_, 1895, Vol. III., p. 252, and Dawson, _Phil.

Trans._, 1900.

CHAPTER XXIV.

EXCRESCENCES (_continued_).

_Cankers--Burrs--Sphaeroblasts, and other excrescences of woody tissues--Witches' Brooms._

_Cankers_ are irregular excrescences due to the perennial struggle between tissues attempting to heal up a wound, and some organism or other agent which keeps the lesion open. A canker always originates in a wound affecting the cambium, and usually in a small wound such as an insect puncture or frost nip; if undisturbed the dead parts would heal over by cork and callus, but if recurring frost-cracks break open the coverings, or if insects or fungi penetrate the callus and invade the cambium, irregularities of growth due to the occluding tissue on the one hand, and continued growth of the still unimpaired cambium on the opposite side of the injured shoot on the other, result in the canker.

Frost cankers occur on fruit-trees, Vines, Beeches, etc.

Cankers due to insects are found on Apples, the cortex of which is punctured by the woolly Aphis (_Schizoneura_) while the twigs are young, and the wound is kept open by the insects nestling in crevices in the occlusion tissues. Species of _Coccus_, _Lachnus_, and _Chermes_ also produce cankers on forest trees.

Cankers due to fungi usually originate in a wound primarily due to an insect puncture or bite, or to frost, the invading fungus hyphae making their way into the wounded tissues and gradually extending more and more into the cambium and the occluding callus. Among the best known of these wound fungi which cause cankers are _Dasyscypha Willkommii_ the peziza of Larch disease, _Nectria ditissima_ and _N. cucurbitula_ on Beech and Conifers; less common are _Scleroderris_ on Willows, _Aglaospora_ on Oaks and some others.

_Peridermium Pini_ and _Aecidium elatinum_ also cause cankers under certain conditions, as also does _Gymnosporangium_, but in these cases the fungi are more truly parasitic.

In some cases--_e.g._ Ash, Pine, Olives--bacteria are concerned as a.s.sociated organisms in the cankering of trees.

_Burrs_ or _Knauers_ are irregular excrescences, princ.i.p.ally woody, with gnarled and warted surfaces. They are frequently due to some previous injury, such as the crushing or grazing of cortical tissues by cart-wheels. The excitation of the tissues thus wounded results in the development of shoots from advent.i.tious or dormant buds at the base of old tree trunks, or in the starting of the same process where a branch has been broken off. The new bud begins to develop a shoot, but soon dies at its tip owing to paucity of food-supplies to the weak shoot, while new buds at its base repeat the process next year with the same result, and each of these again in turn, and so on. The consequence is an extremely complex nest of buds, all capable of growing in thickness and putting on wood to some extent, but not of growing out in length. In course of time this ma.s.s may attain dimensions measurable by feet, forming huge rounded and extremely hard-knotted burrs, the cross-section of which shows the vascular tissues running irregularly in all directions, and, owing to the very slow growth, extremely dense and hard. The dark spots in such sections--_e.g._ Bird's-eye Maple--are the cut bud-axes all fused together, as it were. On old Elms such burrs are common at heights on the stem which preclude the a.s.sumption of any coa.r.s.e mechanical injury, and similar structures occur on the boles of other forest trees suddenly exposed to light by the felling of their companions, which suggests that these epicormic shoots result from some disturbance due to the action of light.

_Witches' Brooms_ are irregular tufts of twigs often found among the branches of trees such as Birches, Hornbeam, etc., where they look like crows' nests, and similar structures are to be found on Silver Firs and other conifers. In the former case they are due to _Exoascus_, in the latter to _Aecidium_, fungi which are perennially parasitic in the shoots, and stimulate the twiggy development of a number of buds which would normally have remained in abeyance, or not have been formed at all, and only do so now in a fashion different from that of normal branches.

Rosette-like formations, depending on similar disturbing causes on the part of insects, occur in conifers--_e.g._ _Gastropacha Pini_.

Dense tufts of twiggy shoots may be developed on many trees by pruning in such a way as to stimulate the shooting out of basal buds which would otherwise remain dormant, _e.g._ Elm, Ash, and thus it occurs that injuries such as frost, insect bites, etc., may induce the production of such tufts in a tree crown. The dense nests of stool-shoots thrown up from felled tree-stumps are of essentially the same nature--partly advent.i.tious and partly dormant buds being enabled to grow out because they can now be supplied with materials previously carried beyond them while the trunk was still there. Suckers, if repeatedly cut down, may also behave similarly.

_Wood-nodules_ or _Sphaeroblasts_ are curious marble-like ma.s.ses of wood which protrude with a covering of bark from old trunks of Beeches, etc., and can be readily dug out with a knife. The nodule has arisen by the slow growth of the cambium of a dormant bud, the base of which separated at an early date from the wood beneath; the cambium then closed in over the base and laid on thickening rings all round the axis of the bud except at the extreme apex. When the separation occurred the cambium of the wood beneath covered over the previous point of junction, and thus the woody bud was pushed out with the bark, and now protrudes covered with a thin layer of the latter. Similar nodules are occasionally found on Apple trees.

NOTES TO CHAPTER XXIV.

For further information on Cankers the student should read Marshall Ward, _Timber and some of its Diseases_, Chapter X.

Further, the discussion as to the causes of canker in Frank, _Krankheiten der Pflanzen_, B. I., p. 207, and B. III., pp.

167 and 172, and various papers in _Zeitschrift fur Pflanzen-krankheiten_.

CHAPTER XXV.

EXUDATIONS AND ROTTING.

_Tumescence--Rankness--Bursting of fruits, etc.--Root rot--Rot of fruits--Bulb diseases--Flux--Honey-dew--Slime flux-- Resinosis--Gummosis--Manna._

I put together in one artificial cla.s.s a varied group of diseases, the princ.i.p.al symptom of which is the escape of fluids from the tissues, under circ.u.mstances which betray an abnormal state of affairs, often obvious, but sometimes only to be inferred. In many of these cases bacteria abound in the putrefying ma.s.s, and some evidence exists for connecting these microbes causally with the disease in a few of the more thoroughly investigated cases, but in no case has this been sufficiently demonstrated; and considering the ease with which bacteria gain access _via_ wounds caused by insects and fungi, as well as by other agents, the necessity for rigid proof must be insisted upon before we can accept such alleged examples of _Bacteriosis_.