402 GENETICS IN RELATION TO AGRICULTURE 
are too strong in alkali content for most plants. Similarly with many 
plant troubles that are referred to adverse soil conditions, such as chlorosis 
and die back, it has been found that some species are much better able to 
resist such conditions than other species and within a particular species 
certain varieties may be more resistant than other varieties. This holds 
true in the case of other non-living agencies such as excess and deficiency 
of moisture and heat. For every plant there is a set of optimum condi- 
tions and these conditions are very different in different species and among 
varieties of the same species. For example, rice flourishes in standing 
water while maize requires well acrated soil. But there are thousands of 
varieties of rice, each one adapted to the conditions peculiar to a certain 
locality and there are many varieties of maize which make possible the 
culture of this species under conditions varying from the humid corn belt 
to the arid regions of northern Mexico, Bolivia and central China. 
Similarly in other field crops and in fruits, in various parts of the world 
there exist species and varieties which are adapted to certain local condi- 
tions that would be inimical to normal development of related species and 
varieties. Agricultural exploration codperating with systematic seed and 
plant introduction has already made available for the plant breeder a 
large number of distinct forms of economic plants which in course of time 
may revolutionize many productive and manufacturing industries. 
Turning now to the phenomena of resistance to the attacks of animal 
or plant parasites, we find that natural species are characterized by as 
great diversity in this respect as was observed in the case of resistance to 
alkali, drouth and other physical elements of the environment. A few 
specific examples will serve to illustrate this general principle. The 
relation of different species of the grape to the phylloxera, Peritymbia 
vitifolie Fitch (Phyllozera vastatriz Planchon), is representative of a great 
number of reported instances of insect parasitism on vegetation. Also 
in their general aspects the phenomena of variation in phylloxera resist- 
ance among species of the vine are representative of the facts of disease 
resistance in general. Moreover, on account of the great economic 
importance which this particular vine disease assumed in Europe some 
forty years ago, and later in California, there has been a large amount of 
investigation on the culture of grapes in phylloxera infested regions. The 
life cycle of this insect includes both leaf-feeding and root-feeding forms. 
The extent of the injury caused by the warty galls on the leaves is com- 
paratively insignificant. It is the root-feeding form which inflicts serious 
damage to susceptible vines. On the roots of such vines the character- 
istic symptoms are of two distinct kinds, viz., small galls or ‘‘nodosities”’ 
near the tips of young rootlets, and larger swellings or ‘‘tuberosities”’ 
occurring upon the older rootlets and roots (Fig. 164). The root-tip galls 
or nodosities are commonly found even on resistant species if phylloxera are 
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