_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, Cucumbers 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_ passes 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. Ass. Rep._, Bradford, 1900. The detailed study of the anatomy and histology of Galls has been recently undertaken by Küster, "_Beitrage zur Kenntniss der Gallenanatomie_," Flora, B. 87, 1900, p. 117, where the principal references will be found.