This section is from the book "The Gardener's Monthly And Horticulturist V22", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
The Gardener's Chronicle says: "As to the powers of resistance to the grape-louse offered by certain of these American varieties (for this precious faculty is not possessed by all), there is no doubt whatever. The fact has been proved in various districts in France, and in particular in one instance, where out of 150,000 cuttings which were put in to replace some vines destroyed by the grape-louse, none are now left, except a few American varieties planted by accident.
" M. Foex has set himself to discover the reasons, for this comparative immunity on the part of the American vines, which he attributes to the structure of their tissues. The roots of the American varieties are stated to have their tissues of a denser and more woody character than those of the European varieties; their medullary rays (silver grain) are also narrower and more numerous. The puncture of the Phylloxera excites a local irritation and swelling which does not pass far beyond the original point of injury, and the traces of which soon disappear. Of course there are variations as to these points according to the particular kind of grape and the nature of the soil".
Investigations on this side of the Atlantic have shown that the American vines are just as subject to attack as the foreign varieties The writer of this has seen roots of Clinton as densely granulated as ever he has seen in the case of any variety. It is not that the American species are less liable to attack, but they suffer less from the the attack. And the reason for this seems to be in the different rooting habits of the species. A careful examination of those liable to injury from the insect attack, shows them to have few long and slender roots; while the other, like Clinton and allies, make innumerable branching fibres. One rootlet is no sooner injured, and its growth checked, than it sends out many more from the main root above. In this easy rooting power lies its strength.