PROPAGATION AND FRUIT-STOCKS 



347 



that there was a faUing off of the importations. This was 

 largely due to scarcity of stock in Europe, as a result of war 

 conditions, together with a prohibitive price placed on this 

 stock. The following figures are enlightening on the large 

 amount of stock imported, following the establishment of 

 quarantine measures by the United States Government: 



Table XCIII 



importation of fruit-stocks, july 1, 1919, to june 30, 1920 

 number of plants 



312. Fruit-trees on their own roots. — Since the fruit- 

 tree above ground is of the variety tlesired, and a part or all 

 of the root system is of seedling origin, it can be seen that 

 considerable variation may be expected among the trees of 

 any given variety. No two of the seedlings used as stock 

 are alike (genetically) and they may vaiy markedly in vigor 

 of growth, susceptibility to disease and insect pest, as well 

 as in hardiness. It is well known to what extent a dwarfing 

 stock may influence the cion part, but the smaller differences 

 arc not readily observed in the standard trees. In Australia, 

 New Zealand, South Africa, and to some extent in California, 

 it is recognized that Northern Spy roots are more resistant 

 to injuiy by woolly aphis (Schizoneura lanigera) than are the 

 ordinary crab roots and hence are finding wide usage. Some 

 varieties are more resistant to crown-gall {Bacterium tume- 

 faciens) than are others, and this is doubtless true also of 

 other diseases, pointing to an important field of endeavor 



