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PACIFIC SCIENCE, Vol. I, July, 1947 
Fig. 4. Green weights and arsenic content of Sudan grass grown in black soil with arsenic content 
ranging from 15 ppm (Can 1) to 3,014 ppm (Can 24) of As 2 Oa. The difference in levels of the two 
green-weight curves is due to the season in which the plants were grown and is not related to treat¬ 
ment. (For As 2 0 3 increments, see Table 10.) 
associated with the depression of growth. 
Sudan Grass: Black Soil .—The growth of 
Sudan grass in black soil, as shown in Table 
10 and Figure 4, was much more luxuriant 
than in the red soil. Furthermore, in the 
black soil normal growth occurred at much 
higher levels of soil arsenic, despite high 
tissue levels of arsenic. Unlike the curve for 
the tomato crops, there was a decided break 
in that of the Sudan grass series. Above 
Can 16 (614 ppm of soil arsenic) growth 
was progressively more difficult, and at the 
soil arsenic level of 2,014 ppm (Can 22) 
there was no growth. 
Although Sudan grass in culture solution 
appeared as tolerant to arsenic as was the 
tomato, it is quite clear that in soils, Sudan 
grass is much less tolerant of arsenic than is 
the tomato. This difference in the soil seems 
only partly related to the ability of Sudan 
grass to extract higher levels of arsenic from 
a given soil. (Compare the arsenic levels in 
Figures 2 and 4 from Cans 13 to 20, in 
which growth of Sudan grass was still appre¬ 
ciable. ) It is also partly related to a differ¬ 
ence in the manner in which the plants hold 
the arsenic within their tissues. Thus, when 
the tomatoes growing in the black soil had 
over 20 ppm within their tissues, their 
growth was nearly normal. At the same tis¬ 
sue levels the growth of Sudan grass was 
nearly stopped. Thus, tolerance to soil 
arsenic involves root tolerance as well as tis¬ 
sue tolerance. Probably, root tolerance is 
