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 aerated 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 cooperating 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 

 vitifolice Fitch (Phylloxera vastatrix 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 



