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 générale 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 associated organisms in the cankering of trees.