May 12,1923 Injury to Foliage by Arsenical Spray Mixtures 
513 
Davis, Gano, and Tetofsky have been conspicuously susceptible, while 
the others have shown no very consistent differences. They are so 
nearly alike that in one test certain ones would be injured a little more 
than the rest, while in other tests other varieties would be most injured. 
For practical purposes they may be considered about alike. It seems 
probable that this varietal susceptibility of apple trees is due to the 
thickness or character of the cuticle on the leaves rather than to a dif¬ 
ference in the living cells within, for we have shown (^o, p. 304) that 
when fresh wounds through the bark are treated with arsenical com¬ 
pounds, Ben Davis is not more injured or Transcendent less injured 
than other varieties. 
Of the common garden beans, four varieties were compared. These 
were the White Navy, Red Kidney, Dwarf Horticultural, and Burpee 
Stringless. In their susceptibility to calcium arsenite, copper aceto- 
arsenite, and London purple they were almost identical. 
From our experiments and those of Woodworth (55) we may conclude 
that among plants of different species, and different varieties of the same 
species, few general rules may be laid down, and each species and variety 
must be actually tested in comparison wiA others to know its suscep¬ 
tibility. To be sure, some groups, as beans, are especially susceptible 
(probably all of them) and probably all cabbages are relatively resistant. 
But among plants of intermediate susceptibility such as apple, cherry, 
rose, potato, and sugar beet, varietal diference is sufficient to make it 
uns^e to draw comparisons between these species without stating the 
varieties. 
INDIVIDUAL DIFFERENCES 
Do individuals of the same species and variety show differences in 
susceptibility? If such be the case, general conclusions can not be 
drawn with safety from single tests, and some repetition is necessary to 
get the prevailing tendency, the amount of repetition depending upon 
the degree and frequency of individual variation and the exactness of 
the method used. 
In carrying on this investigation, practically all the herbaceous 
plants used, whether in the greerdiouse or in the field plots, were sprayed 
in triplicate and in some cases three branches of as many different 
orchard trees were sprayed in the same way. Opportunity was thereby 
offered for a very large number of comparisons. The results showed that 
the tendency to individual variation is quite marked. As this tendency 
is a matter of degree and we have no standards for expressing it, reference 
may be made to Table IV, which is quite representative of this condition. 
(See also Tables IX to XIV.) Doubtless this variation is partly apparent 
and partly real. With the greatest of care it is difficult to keep a large 
number of plants under identical conditions after spraying. One ma> 
be kept in a little more shaded place than another, or have better air 
circulation, or be brushed more in working among the plants. This 
difficulty is even greater in treating branches of orchard trees. Using 
every precaution to eliminate such factors, individual differences still 
appeared sufficient in degree to convince one that the triplicate plants 
at the time of spraying were not always alike in susceptibility. We be¬ 
lieve that this is due in part to the environmental conditions under which 
the plants were grown, though all showing visible abnormalities were 
rejected before spraying, and there also may be a natural tendency to 
more or less resistant strains. 
