As a third illustration I will take the case of an insect epidemic. In 1863 a disease was observed on vines in the South of France which frightened the growers as they realised its destructive effects: the roots decayed and the leaves turned yellow and died before the grapes ripened, and such vines threw out fewer and feebler shoots the following year, and often none at all afterwards. In 1865 the disease was evidently becoming epidemic near Bordeaux, and in 1868 it was shown to be due to an insect, Phylloxera, the female of which lays its eggs on the roots, where they hatch. The louse-like offspring sticks its proboscis into the tissues as far as the central cylinder. The irritated pericycle and cortex then grow and form nodules of soft juicy root-tissue at which the insect continues to suck. Rapid reproduction results in the majority of the young rootlets being thus attacked, and since they cannot form their normal periderm and harden off properly they rot, and admit fungi and other evils, in consequence of which the vine suffers also in the parts above ground.
Evidence that the general damage is due to the diminished root-action is found in the peculiarly dry poor wood formed in the "canes" of diseased plants.
By 1877 the epidemic had spread to the northern limits of the French vineyards, and by 1888 half the vines in the country were attacked, and the yield of wine reduced from half a million hectolitres to 50,000 only. Meanwhile the disease had spread to Italy, Germany, Madeira, Portugal, and even to the Cape, though not in epidemic form as in the Bordeaux centre whence it spread.
Now it appears that Phylloxera has long been in the habit of doing damage to vines in America, where, however, it attacks the leaves, on which it makes pocket-like galls, rather than the roots. Moreover, there are species and varieties of American vines which, even when planted in Europe, do not suffer at all from this insect at the roots, either because the rootlets do not push out at the same season as those of the European form, or because they form wood more rapidly and completely, or secrete resinous and other matters distasteful to the insect in greater quantity and are thus capable of healing the wounds, or in some other way they do not respond to the attack or suit the insect. In any case the attack on the leaf rather than the root seems to be the exception in European vineyards and the rule in American species, and we appear to be face to face with a problem of specific predisposition to this particular malady. That the resistant properties of the vines of America - not all, only particular species and varieties are thus "immune" - can be utilised has been proved by European growers; and not only so, for Millardet and others have shown that the European vine grafted on to these resistant stocks suffer less than when on their own roots. It has also been shown that hybrids can be obtained which are resistant.
But the most curious point of all is that Phylloxera was itself a native of America, and came thence to Europe. It had played its part with certain fungi in ruining all the attempts to introduce the European vine into America many years ago. A recent authority on the evolution of American fruits writes as follows:
"All the most amenable types of grapes had long since perished in the struggle for existence, and the types which now persist are necessarily those which are, from their very make-up or constitution, almost immune from injury, or are least liable to attack . . . the Phylloxera finds tough rations on the hard, cord-like roots of any of our eastern species of grapes. But an unnaturalised and unsophisticated foreigner, being unused to the enemy and undefended, falls a ready victim; or if the enemy is transported to a foreign country the same thing occurs."
Further proof that it is in the "constitution" of the European vine that the want of resistance to Phylloxera resides, is furnished by the fact that in California and the Pacific states the European vine was introduced with more success, but is now suffering badly because Phylloxera has spread there also. It must not be overlooked, however, that we are as yet very ignorant of all that is implied in the word "constitution" as used above.
If we enquire further why the Phylloxera epidemic was so much worse in the Southern vineyards than in the more Northern ones of Germany, the opinion seems to prevail that the warmer climates favour the insect. Further, it appears that, in Italy, the vines in loose open soil, provided it is equally rich in mineral food-materials and offers no disadvantages as regards drainage, suffer less than those in closer soils, the reasons alleged being that the young roots can push out more rapidly and widely, and so obtain holdfasts with greater distances between them.
The student may obtain further information on the history of the Potato disease by consulting the following: Berkeley, "Observations, Botanical and Physiological, on the Potato Murrain," Journal of the Horticultural Society, Vol. I., 1846, p. 9; De Bary, Die Gegenwartig herrschende Kartoffel Krankheit, etc., Leipzic, 1861; and the pages of the Gardeners' Chronicle from 1860 - 1900.
For the Larch disease he should consult Hartig, Unters. aus der Foist. Botanischen Inst. Munchen, B. I., 1880; and Willkomm, Microscop. Feinde des Waldes, B. II., 1868.
For Phylloxera the literature is chiefly in the Co7nptes Rendus and other French publications since 1875, and in the Reports of the U.S. Dept. of Agriculture.
For a summary of the facts concerning the life-histories of the parasites referred to above, see Frank, Krankheiten der Pflanzen, and Marshall Ward, Diseases of Plants, p. 59, and Timber and Some of its Diseases, London, 1889, chapter X.
Also Marshall Ward, "On some Relations between Host and Parasite in certain epidemic Diseases of Plants," Proc. Roy. Soc., Vol. XLVI I., 1890, pp. 393 - 443; and "Illustra tions of the Structure and Life-history of Phytophthora infestans," Quart. Journ. Microsc. Soc., Vol. XXVII., 1887, p. 413; also Marshall Ward, "Researches on the Life-history of Hemileia vastratrix," Journ. Linn. Soc., Vol. XIX., 1882, p. 299; and "On the Morphology of Hemileia vastatrix," Quart. Journ. Microsc. Soc., 1881, Vol. XXI., p. 1.