BREEDING DISEASE-RESISTANT PLANTS 403 
present. The principal difference between resistant and susceptible vines 
as regards reaction to phylloxera attacks is found in the number, size and 
penetration of the lesions on the larger roots. This phylloxeran is a 
native of eastern North America and the species of Vitis which are native 
to this region all exhibit some resistance to its attacks. This resistance 
of species native to the habitat of a disease-causing parasite is a general 
fact of great significance to agriculture on account of its potential value 
in both plant and animal breeding. 
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Fia. 164.—Effects of phylloxera on vine roots. On left affected root tips or nodosities; 
in same figure incipient tuberosities are shown at a. Center, non-penetrating tuberosities 
on an American vine. Right, penetrating and confluent tuberosities on V. vinifera, the 
most serious form of the disease. (After Viala and Ravaz.) 
The phylloxera was introduced into France through the importation 
of American vines and it soon became a most serious obstacle to the 
culture of the choice wine, table and raisin grapes of the Mediterranean 
region, all of which varieties belong to a single species, Vites vinifera. 
In fact, every member of this large and valuable plant group has been 
found to be susceptible to phylloxera thus making impossible its culture 
as a direct producer, 7.c., on its own roots, in a phylloxera infested region. 
After striving in vain to exterminate the insect in all infested areas, 
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