LUTHER BURBANK 
of this variety are never injured by these insects, 
even when planted beside trees seriously infected. 
It has been found expedient, therefore, to graft 
other varieties on roots of the Northern Spy, and 
an orchard that has practical immunity to the at- 
tacks of the aphis may thus be produced. 
Unfortunately the seedlings of the Northern 
Spy do not generally inherit this quality of resist- 
ance to the aphis, so it is necessary to grow the 
roots from cuttings. 
Apple twigs do not root very readily, but if 
cuttings from vigorous Northern Spy branches 
are placed in the soil and allowed to grow for a 
year or longer they develop a good root system 
and the roots may be severed into small pieces, 
each of which will produce a stock upon which 
grafts of any variety may be placed. 
Hysrip APPLES 
I have experimented very extensively, as al- 
ready noted, with the crossing of different familiar 
varieties of apple, and have produced several new 
varieties that have been deemed worthy of intro- 
duction. 
But my most interesting experiments have had 
to do with the wider hybridization in which one 
or another variety of cultivated apple has been 
crossed with a related species. In endeavoring 
to introduce new traits I imported in 1890 all of 
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