252 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



barriers can be effective, wfiile with some virus and other diseases 

 which are spread by insects, the kilhng of these vectors, or growth 

 of the crop in areas where they do not occur, should suffice for 

 successful control. Some plant pathogens have an alternative host 

 in which part of the life-cycle is spent. In such cases eradication 

 of this alternative host is effective, an example being afforded by 

 the White Pine Blister-rust, which threatens the life of all five- 

 needled Pines in North America, and whose alternative host is the 

 genus Ribes (Currants and Gooseberries). It should be noted, 

 however, that such systematic eradication may not merely affect the 

 distribution of the species concerned but also the local ecological 

 balance, which may be seriously upset. Similarly, DDT spraying 

 against harmful insects may at the same time kill off all the bees 

 which are necessary for pollination! 



Further Consideration 



E. V. WuLFF. An Introduction to Historical Plant Geography (Chronica 

 Botanica, Waltham, Mass., pp. xv + 223, 1943) ; for further details 

 about many of the topics discussed in the early part of this chapter. 



Origins of Crops : 



Alphonse DeCandolle. Origin of Cultivated Plants (Kegan Paul & 

 Trench, London, pp. ix 4- 468, 1884) '■> still useful. 



N. I. Vavilov. The Origin, Variation, Immunity and Breeding of 

 Cultivated Plants, translated by K. S. Chester (Chronica Botanica, 

 Waltham, Mass., vol. 13, Nos. 1-6, pp. xviii + 364, 1951); see also 

 below. 



Edgar Anderson. Plants, Man and Life (Little, Brown, Boston, Mass., 

 pp. [vii] + 245, 1952) ; stimulating. 



E. D. Merrill. The Botany of Cook's Voyages (Chronica Botanica, 

 Waltham, Mass., vol. 14, Nos. 5-6, pp. i-iv -|- 161-384, 1954) ; for 

 additional information, with a master's pungent criticisms of some 

 previous contentions. 



For Details about Herbaceous Crops : 



E. E. Stanford. Economic Plants (Appleton-Century-Crofts, New York, 



pp. xxiii + 571, 1934). 

 K. H. W. Klages. Ecological Crop Geography (Macmillan, New York, 



pp. xviii + 615, 1942). 

 A. F. Hill. Economic Botany, second edition (McGraw-Hill, New York 



etc., pp. xii + 560, 1952). 



