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obvious problems of growing this tree under experimental conditions. No
results are as yet available of physiological studies of the factors which
control the absorption of radium in other species. The comparative study
of radium and the other alkaline earths, magnesium, calcium, strontium,
and barium, would appear to hold promise of elucidating some aspects of
the mechanisms whereby cations are absorbed and transferred in plants.
Lead 210 and polonium 210.—Lead 210 (half life 19 years) and polonium
210 (half life 138 days) are the only two radioactive substances of appreciable half life which arise from radon 222, the gaseous emanation of radium.
The polonium isotope, unlike lead 210, emits alpha radiation, and has been
shown to be the major component of the alpha activity in grass grown
under normal field conditions (89). It has been suggested that this is mainly
due to the deposition in rain of the decay products of radon which escape
from the earth’s crust into the atmosphere. Other investigations provide
evidence of the absorption of the lead isotope from the soil (2).
Carson 14
Carbon 14, both natural and man-made, is formed in the atmosphere
from nitrogen. Its natural occurrence is due to the action of cosmic rays;
man-made carbon 14 results from the release of neutrons, formed mainly
in nuclear weapons tests. The relative amounts in which carbon 14 and
fission products arise from nuclear weapons depends on the ratio of total
yield (fission plus fusion) to fission yield. Thus, carbon 14 will become
proportionately more significant if larger or “cleaner” weapons are ex-
ploded; the adjective “clean” refers to the extent of fission only. Although
carbon 14 enters into biological systems through photosynthesis, the evalua-
tion of its effects presents few problems for the plant physiologist since the
specific activity of organic carbon will reflect that of the atmospheric carbon dioxide from whichit is derived.
The changes in the specific activity of atmospheric carbon dioxide
because of nuclear weaponstrials may, however, provide an important tool
for certain types of biological research. The work of Libby (90) has caused
the general principles of “‘carbon-dating” to become widely known. Until
considerable releases of carbon dioxide from fossil sources occurred in
the late 19th century the specific activity of atmospheric carbon dioxide is
believed to have been constant for a considerable period; thus the date at
which organic carbon was synthesised could be calculated from its specific
activity on the basis of the half life of carbon 14 (5570 years). In the
recent past, however, the specific activity of the atmospheric carbon dioxide
has increased. In 1955 it was estimated that the mean value in the troposphere
of the Northern Hemisphere was a few per cent above the natural level, but
in 1959 it was enhanced to nearly 30 per cent (2). Further increases are to be
expected. Accordingly, the specific activity of carbon in organic compounds
which have been formed by photosynthesis in the last decade will vary considerably depending on their date of formation. Thus, whereas “carbon-
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