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RUSSELL
tively different relationships must be expected under contrasting conditions
of climate and husbandry. However, as broad similarities between the
pattern of contamination of pastures in North America and Europe are
suggested by many results of surveys (2, 27), the situation in the United
Kingdom is unlikely to be unique.
Cereals—Enquiry into the mechanism whereby strontium 90 enters cereal
grains from world-wide fallout was first encouraged by the observation that
the ratio of strontium 90 to calcium in flour might be two or more times
higher than that in leaf vegetables or the majority of other foods, while
that in whole grain was larger still, often about three times that in flour
(2). Clear evidence that strontium 90 entered grain largely by direct floral
contamination was provided by a comparison of the ratios of strontium 90
from world-wide fallout to stable strontium in the different tissues of cereal
ears (41). As the soil is the sole source of stable strontium, the ratio of
strontium 90 to stable strontium in all tissues formed at the same time would
be constant if strontium 90 entered only from the soil. Variations in this
ratio thus indicate direct contamination with strontium 90 unaccompanied
by stable strontium. The analysis of the different tissues of grain harvested
in the United Kingdom and in other countries in 1957 showed that the ratio
of strontium 90 to stable strontium in flour was always considerably lower
than that in other tissues, sometimes by a factor exceeding 10 (41). A
minimum estimate of the extent to which strontium 90 had entered by
direct contamination could be made by assuming that all strontium 90 in the
flour came from the soil. On this basis it appeared that at least two-thirds
of the strontium in grain was due to direct contamination; when results
for later years became available the corresponding estimate was about 50
per cent (5). Comparable calculations for different parts of the United
States gave values of 20 and 90 per cent in 1959 (42, 43).
The importance of floral contamination has also been demonstrated in
laboratory experiments, If strontium 89 is applied as a spray after ear
emergence the contamination of the grain at harvest may be some 15 times
greater than if the same application is made shortly before the ears emerge
and the protection of emerged ears with small caps reduces contamination
to the level found when the spray is applied before ear emergence (32).
Other crops.—Laboratory experiments have been carried out in which
strontium 89 has been applied in a fine spray to cabbages, potatoes, and
sugar beet at different stages of their growth (32, 45). Since the edible
heart tissues of cabbages are protected by outer leaves and strontium 90
does not move basipetally within plants, the manner in which they are
“directly” contaminated is not altogether obvious. The occurrence of direct
contamination of the heart tissues, though to a much smaller extent than in
pasture grass, is however well established both by experimental studies and
surveys of world-wide fallout (41). It has been suggested that this may be
due to strontium 90 being carried down in rain and lodging in the saucer-
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