VARIATION 
531 
including from fifteen to twenty-five different species, about 
one half of the total number known to man. These different 
kinds of grapes grow in varied areas and climates. Their 
original distribution over North America was due to animals, 
such as birds, and to river currents. 
The several varieties of American grapes are descendent 
from an original species which has become changed by 
environment until we find 
types of grapes as diverse 
as the regions in which 
they are found. 
Many of these varieties 
have been in existence 
since white men came 
to this country. During 
this time they have ac¬ 
quired a strong resistance 
to certain parasitic dis¬ 
eases which are very de¬ 
structive to the European 
grapes. Among them is 
one that is caused by a 
minute plant louse that 
lives either in the leaf, 
stem, or root. This in¬ 
sect is known as phyllox¬ 
era (fil-oks-e'ra) and is 
very destructive. In addition to this animal disease, there 
are three plant parasite diseases: black-rot, downy mildew, 
and powdery mildew. Each of these four diseases is able to 
kill grape plants and more than one may attack the same 
plant at the same time. 
A few years ago these diseases became widespread in 
Europe and threatened to destroy most of the European 
vineyards. Many experiments were undertaken to save 
Figure 438 . — Leaf Galls on Grape 
Caused by Grape Phylloxera. 
The roots are also injured by this dis¬ 
ease which threatened to destroy all of 
the European vineyards. 
