404 GENETICS IN RELATION .TO AGRICULTURE 
European vineyardists gradually adopted the only other practicable 
method of grape growing, viz., the grafting of vinifera varieties upon 
resistant roots. The problem of determining which species of Vitis were 
both highly resistant to phylloxera and well adapted to the soil and 
climatic conditions of various European localities required extensive 
investigations. Eighteen native American grapes have been tested as 
well as several Asiatic species, but the latter were all less resistant than 
the most susceptible American species. The American vines which 
have come into most prominence on account of their proven value in 
the reconstitution of phylloxera devastated vineyards may be listed 
according to relative resistance about as follows, if the maximum or absolute 
immunity be taken as 20. 
18-19. V. rupestris. 
18. V. r¢paria and cordifolia. 
ike V. berlandiert. 
16. V. crnerea. 
14-15. V. estivalis, linsecomar and candicans. 
All of the above species belong to the sub-genus or section, Euvitis. 
Two of these, rupestris and riparia, together with certain hybrids 
between these and between these and vinifera, are now considered the 
most valuable resistant stocks. Another American species belonging to 
the section Muscadinia, viz., rotundifolia, has been found to have a 
resistance of 19 or higher inasmuch as the insect has never been observed 
on its roots. It is also free from the common fungous diseases of the 
vine, but the difficulty of propagating it from cuttings and its slight 
affinity for grafts of other species make it a valueless species for the 
reconstitution of vineyards. On the other hand, the American species, 
labrusca, has become of great economic importance since it is the parent 
of the Concord, Isabella, Niagara and many other cultivated varieties. 
Yet its resistance to phylloxera is ranked at 5, and when grown in Cali- 
fornia it is no more resistant than is californica when used as a rootstock 
for producing vines, and the resistance of the latter is ranked at 4. Yet 
the labrusca derivatives are extensively grown in the northeastern states 
and in other northern temperate regions. This is explained by the fact 
that the phylloxera itself does not thrive below a certain minimum 
temperature. Thus we find that resistance to phylloxera in the species 
of Vitis varies all the way from zero in vinifera to practically absolute 
resistance in rupestris, rotundifolia and certain hybrids and that the 
existence of highly resistant forms which are also suitable for vineyard 
culture has made possible the preservation of an important agricultiiral 
industry. 
The question of the nature of the cause of resistance to phylloxera 
has received rather wide attention among investigators, but it has not 
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